pytest static analysis (a) suppress coverage warning for cv2, (b) remove one poor unit test.

[ruck94301]

Use an improved approach to suppress cv2 coverage warning.

Previously, avoided nuisance warnings during static analysis with pytest
"coverage" reporting by suppressing the import of opencv-python during
pytest, using an environment variable setting that detectron2 honors.

With this PR, revert to normal import of cv2 (that is, opencv-python),
and use a better workaround to suppress the two coverage warnings, namely
filterwarnings via setting in pytest.ini.

Also pytest related,
remove John's misguided unit test which attempts to produce an
import-duration.

1. It doesn't work reliably, because the result can be improved by
   performing another test case earlier in the pytest session, if the
   other test case loads libraries... which means this test case
   doesn't lose time loading.
2. It's not necessary. To get a measure of import duration, use
   python's "-X importtime". This is a feature introduced in
   Python 3.7.
   Documented in new
   LabGym/tests/Notes.importtime.txt