test: Fix for import errors on plugins when executing isolated tests

[AmeyaVS]

• import test_settings for fails in certain pytest invocation when running locally
  • The failing scenario is when triggering to execute individual tests:

  pytest --rootdir=$PWD package/test/test_VersionedPlugin.py

• Replaced TEST_MESSAGE with logging.debug as per the implementation in test_settings.py

Snippet of the error when using the above mentioned pytest command:

ERROR    yapsy:PluginManager.py:518 Unable to import plugin: /home/ameya/plugins/yapsy/package/test/plugins/VersionedPlugin12a1
Traceback (most recent call last):
  File "/home/ameya/plugins/yapsy/package/yapsy/PluginManager.py", line 515, in loadPlugins
    candidate_module = PluginManager._importModule(plugin_module_name, candidate_filepath)
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ameya/plugins/yapsy/package/yapsy/PluginManager.py", line 587, in _importModule
    spec.loader.exec_module(candidate_module)
  File "<frozen importlib._bootstrap_external>", line 995, in exec_module
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "/home/ameya/plugins/yapsy/package/test/plugins/VersionedPlugin12a1.py", line 8, in <module>
    from test_settings import TEST_MESSAGE
ModuleNotFoundError: No module named 'test_settings

[coveralls]

coverage: 91.461% (-0.03%) from 91.489%
when pulling ee8cbea on AmeyaVS:fix_isolated_test_run
into 6b487b0 on tibonihoo:master.