Session scoped fixtures can leak out of a class

Best explained with a failing test, see:

https://github.com/jinty/pytest/commit/c0048b115314dd4a54d1fea5032d85f52efd221b

or inline for the lazy:

import pytest

@pytest.fixture(scope='session')
def foo():
    return 'foo'

@pytest.fixture(scope='session')
def bar(foo):
    return foo + 'bar'

@pytest.fixture(scope='session')
def baz(bar):
    return bar + 'baz'

class TestSpecific:

    @pytest.fixture(scope='session')
    def foo(self):
        return 'FOO'

    @pytest.fixture(scope='session')
    def baz(self, bar):
        return bar + 'BAZ'

    def test_b(self, foo, bar, baz):
        pass

def test_c(foo, bar, baz):
    assert foo == 'foo'
    assert bar == 'foobar'
    assert baz == 'foobarbaz'

Fails with the current master.

Start by running the inline Python reproducer, or inspect commit c0048b115314dd4a54d1fea5032d85f52efd221b, to trace how session-scoped fixtures defined inside TestSpecific affect later tests. Done means test_c receives the module-level foo, bar, and baz values and its assertions pass without breaking the class-scoped fixture behavior.