Output information during the test run

Depending on test runtime and repetition count flaky tests can take a long time to complete.

It would be nice if there was some feedback on flakyness during the test run, not only afterwards.

For now we're using the following as a slightly hacky workaround but a built in solution would be much nicer:
```python
@pytest.hookimpl(hookwrapper=True, trylast=True)
def pytest_runtest_call(item):
""" More feedback for flaky tests.

In verbose mode this outputs 'FLAKY' every time a test marked as flaky fails.
This doesn't work under xdist and will therefore show no output.
"""
outcome = yield
did_fail = isinstance(outcome._excinfo, tuple) and isinstance(
outcome._excinfo[1], BaseException
)
is_xdist = "PYTEST_XDIST_WORKER" in os.environ
is_flaky_test = item.get_closest_marker("flaky") is not None
if did_fail and is_flaky_test and not is_xdist:
if item.config.option.verbose > 0:
capmanager = item.config.pluginmanager.getplugin("capturemanager")
with capmanager.global_and_fixture_disabled():
item.config.pluginmanager.get_plugin("terminalreporter")._tw.write(
f"FLAKY ", yellow=True
)
```