thread-unsafe test failures are hard to spot

[crusaderky]

If a test tagged with pytest.mark.thread_unsafe fails, there is a FAILED tag on the line of the test. There is no traceback. At the end of the test suite, the summary reports that all tests passed. Finally, pytest exits 1.
When the test suite is very large, one has to comb through the pytest stdout to find which test failed.

@pytest.mark.thread_unsafe
def test1():
    assert False

$ pytest -v --parallel-threads=4 test1.py:
=============================================================== test session starts ===============================================================
test1.py::test1 FAILED ([thread-unsafe]: uses thread_unsafe marker)                                                               [100%]

*********************************************************** pytest-run-parallel report ************************************************************
1 tests were not run in parallel because of use of thread-unsafe functionality, to list the tests that were skipped, re-run while setting PYTEST_RUN_PARALLEL_VERBOSE=1 in your shell environment
======================================================== 1 passed, 258 deselected in 0.17s ========================================================
$ echo $?
1

Compare to plain pytest:

$ pytest -v test1.py
tests/test_lazy.py::test1 FAILED                                                                                                            [100%]

==================================================================== FAILURES =====================================================================
______________________________________________________________________ test1 ______________________________________________________________________

    @pytest.mark.thread_unsafe
    def test1():
>       assert False
E       assert False


test1.py:3: AssertionError
============================================================= short test summary info =============================================================
FAILED tests/test_lazy.py::test1 - assert False
======================================================== 1 failed, 258 deselected in 0.15s ========================================================

Or pytest-run-parallel on an unmarked test:

$ pytest -v --parallel-threads=4 test1.py:
test1.py::test1 PARALLEL FAILED                                                                                                   [100%]

===================================================================== ERRORS ======================================================================
_____________________________________________________________ ERROR at call of test1 ______________________________________________________________

    def test1():
>       assert False
E       assert False


test1.py:2: AssertionError
*********************************************************** pytest-run-parallel report ************************************************************
All tests were run in parallel! 🎉
============================================================= short test summary info =============================================================
PARALLEL FAILED test1.py::test1 - assert False

[lysnikolaou]

Opened #71 to fix this.