filelock_test does not catch faulty lock

[TheMatt2]

There is a potential issue in test_filelock.py

If someone where to introduce a bug (such as reintroducing #31) that caused file locking
not to secure exclusive access, no test actually fails. Instead, a warning is shown by Pytest:

(do to the nature of this kind of bug, it seems sometimes this would succeed with no warning at all)

tests/test_error.py ....                                                                                                                                      [  8%]
tests/test_filelock.py ..............................................                                                                                         [100%]

========================================================================= warnings summary ==========================================================================
tests/test_filelock.py::test_threaded_lock_different_lock_obj[UnixFileLock]
  .../site-packages/_pytest/threadexception.py:73: PytestUnhandledThreadExceptionWarning: Exception in thread t2_8
  
  Traceback (most recent call last):
    File ".../threading.py", line 1038, in _bootstrap_inner
      self.run()
    File "py-filelock/tests/test_filelock.py", line 187, in run
      super().run()
    File ".../threading.py", line 975, in run
      self._target(*self._args, **self._kwargs)
    File "py-filelock/tests/test_filelock.py", line 234, in t_2
      assert not lock_1.is_locked
  AssertionError: assert not True
   +  where True = <filelock._unix.UnixFileLock object at 0x1030ef3d0>.is_locked
  
    warnings.warn(pytest.PytestUnhandledThreadExceptionWarning(msg))

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=================================================================== 50 passed, 1 warning in 3.53s ===================================================================

Results found by adding the follow to py-filelock/src/filelock/_unix.py > UnixFileLock > _release

59            try:
60                os.remove(self._lock_file)
61            except OSError:
62                pass

The issue seems to be that the test test_filelock.py::test_threaded_lock_different_lock_obj[UnixFileLock] is failing with a AssertionError exception. However, only RuntimeError's are caught in threads created during the testing process.

https://github.com/tox-dev/py-filelock/blob/b815134c9bc62aecbe2d8b85a875690d55df5667/tests/test_filelock.py#L185-L189