Flaky, xdist, and fixture exceptions can result in test suites running indefinitely

This happens if and only if all of the following are true:
* using xdist
* using flaky
* An exception occurs in a fixture
* There are remaining tests left to be assigned to processes, and every process has died due to the exceptions occurring in fixtures.

Sample test script:
```
import pytest
import flaky

@pytest.fixture
def fixture():
raise AssertionError

@pytest.mark.usefixtures('fixture')
@flaky.flaky(max_runs=2, min_passes=1)
class TestCase:

@pytest.mark.foobar
def test_foo(self):
pass

def test_bar(self,):
pass

def test_baz(self):
pass
```

Sample output:

```
[dshuga@i-110516 ~/indeed/shield/products/examples master]
(shield) $ pytest -v -n1 test_example.py
================================================================== test session starts ===================================================================
platform darwin -- Python 3.5.1, pytest-3.0.5, py-1.4.31, pluggy-0.4.0 -- /Users/dshuga/.pyenv/versions/shield/bin/python
cachedir: .cache
rootdir: /Users/dshuga/indeed/shield/products/examples, inifile:
plugins: indeed-shield-0.0.0, flaky-3.3.0, warnings-0.1.0, xdist-1.15.0
[gw0] darwin Python 3.5.1 cwd: /Users/dshuga/indeed/shield/products/examples
[gw0] Python 3.5.1 (default, May 4 2016, 14:23:33) -- [GCC 4.2.1 Compatible Apple LLVM 6.1.0 (clang-602.0.53)]
gw0 [3]
scheduling tests via LoadScheduling

test_example.py::TestCase::test_foo
```

(stays this way indefinitely)

```
[dshuga@i-110516 ~/indeed/shield/products/examples master]
(shield) $ pytest -v -n3 test_example.py
================================================================== test session starts ===================================================================
platform darwin -- Python 3.5.1, pytest-3.0.5, py-1.4.31, pluggy-0.4.0 -- /Users/dshuga/.pyenv/versions/shield/bin/python
cachedir: .cache
rootdir: /Users/dshuga/indeed/shield/products/examples, inifile:
plugins: indeed-shield-0.0.0, flaky-3.3.0, warnings-0.1.0, xdist-1.15.0
[gw0] darwin Python 3.5.1 cwd: /Users/dshuga/indeed/shield/products/examples
[gw1] darwin Python 3.5.1 cwd: /Users/dshuga/indeed/shield/products/examples
[gw2] darwin Python 3.5.1 cwd: /Users/dshuga/indeed/shield/products/examples
[gw0] Python 3.5.1 (default, May 4 2016, 14:23:33) -- [GCC 4.2.1 Compatible Apple LLVM 6.1.0 (clang-602.0.53)]
[gw1] Python 3.5.1 (default, May 4 2016, 14:23:33) -- [GCC 4.2.1 Compatible Apple LLVM 6.1.0 (clang-602.0.53)]
[gw2] Python 3.5.1 (default, May 4 2016, 14:23:33) -- [GCC 4.2.1 Compatible Apple LLVM 6.1.0 (clang-602.0.53)]
gw0 [3] / gw1 [3] / gw2 [3]
scheduling tests via LoadScheduling

test_example.py::TestCase::test_baz
test_example.py::TestCase::test_bar
test_example.py::TestCase::test_foo
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR> File "/Users/dshuga/.pyenv/versions/shield/lib/python3.5/site-packages/_pytest/main.py", line 98, in wrap_session
INTERNALERROR> session.exitstatus = doit(config, session) or 0
INTERNALERROR> File "/Users/dshuga/.pyenv/versions/shield/lib/python3.5/site-packages/_pytest/main.py", line 133, in _main
INTERNALERROR> config.hook.pytest_runtestloop(session=session)
INTERNALERROR> File "/Users/dshuga/.pyenv/versions/shield/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 745, in __call__
INTERNALERROR> return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
INTERNALERROR> File "/Users/dshuga/.pyenv/versions/shield/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 339, in _hookexec
INTERNALERROR> return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR> File "/Users/dshuga/.pyenv/versions/shield/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 334, in
INTERNALERROR> _MultiCall(methods, kwargs, hook.spec_opts).execute()
INTERNALERROR> File "/Users/dshuga/.pyenv/versions/shield/lib/python3.5/site-packages/_pytest/vendored_packages/pluggy.py", line 614, in execute
INTERNALERROR> res = hook_impl.function(*args)
INTERNALERROR> File "/Users/dshuga/.pyenv/versions/3.5.1/envs/shield/lib/python3.5/site-packages/xdist/dsession.py", line 536, in pytest_runtestloop
INTERNALERROR> self.loop_once()
INTERNALERROR> File "/Users/dshuga/.pyenv/versions/3.5.1/envs/shield/lib/python3.5/site-packages/xdist/dsession.py", line 555, in loop_once
INTERNALERROR> call(**kwargs)
INTERNALERROR> File "/Users/dshuga/.pyenv/versions/3.5.1/envs/shield/lib/python3.5/site-packages/xdist/dsession.py", line 593, in slave_slavefinished
INTERNALERROR> assert not crashitem, (crashitem, node)
INTERNALERROR> AssertionError: ('test_example.py::TestCase::()::test_bar', )
INTERNALERROR> assert not 'test_example.py::TestCase::()::test_bar'

============================================================== no tests ran in 1.39 seconds ==============================================================

[dshuga@i-110516 ~/indeed/shield/products/examples master]
(shield) $
```

```
[dshuga@i-110516 ~/indeed/shield/products/examples master]
(shield) $ pytest -v -n1 -p no:flaky test_example.py
================================================================== test session starts ===================================================================
platform darwin -- Python 3.5.1, pytest-3.0.5, py-1.4.31, pluggy-0.4.0 -- /Users/dshuga/.pyenv/versions/shield/bin/python
cachedir: .cache
rootdir: /Users/dshuga/indeed/shield/products/examples, inifile:
plugins: indeed-shield-0.0.0, warnings-0.1.0, xdist-1.15.0
[gw0] darwin Python 3.5.1 cwd: /Users/dshuga/indeed/shield/products/examples
[gw0] Python 3.5.1 (default, May 4 2016, 14:23:33) -- [GCC 4.2.1 Compatible Apple LLVM 6.1.0 (clang-602.0.53)]
gw0 [3]
scheduling tests via LoadScheduling

test_example.py::TestCase::test_foo
[gw0] ERROR test_example.py::TestCase::test_foo
test_example.py::TestCase::test_bar
[gw0] ERROR test_example.py::TestCase::test_bar
test_example.py::TestCase::test_baz
[gw0] ERROR test_example.py::TestCase::test_baz

========================================================================= ERRORS =========================================================================
__________________________________________________________ ERROR at setup of TestCase.test_foo ___________________________________________________________
[gw0] darwin -- Python 3.5.1 /Users/dshuga/.pyenv/versions/shield/bin/python
@pytest.fixture
def navigate_to_indeed():
> raise AssertionError
E AssertionError

test_example.py:14: AssertionError
__________________________________________________________ ERROR at setup of TestCase.test_bar ___________________________________________________________
[gw0] darwin -- Python 3.5.1 /Users/dshuga/.pyenv/versions/shield/bin/python
@pytest.fixture
def navigate_to_indeed():
> raise AssertionError
E AssertionError

test_example.py:14: AssertionError
__________________________________________________________ ERROR at setup of TestCase.test_baz ___________________________________________________________
[gw0] darwin -- Python 3.5.1 /Users/dshuga/.pyenv/versions/shield/bin/python
@pytest.fixture
def navigate_to_indeed():
> raise AssertionError
E AssertionError

test_example.py:14: AssertionError
================================================================ 3 error in 1.49 seconds =================================================================

[dshuga@i-110516 ~/indeed/shield/products/examples master]
(shield) $
```

Probably the same root cause as https://github.com/box/flaky/issues/114