Add unit tests in preparation for moving the aaf adapter

[markreidvfx]

This pull request should keep all the coverage lost if #1348 was merged

[codecov-commenter]

Codecov Report

Merging #1353 (4648de5) into main (f54569f) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1353      +/-   ##
==========================================
+ Coverage   86.18%   86.27%   +0.08%     
==========================================
  Files         196      196              
  Lines       19825    19865      +40     
  Branches     2309     2309              
==========================================
+ Hits        17087    17138      +51     
+ Misses       2171     2161      -10     
+ Partials      567      566       -1     

Flag	Coverage Δ
py-unittests	86.27% <100.00%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tests/test_clip.py	97.80% <100.00%> (+0.07%)	⬆️
tests/test_effect.py	96.07% <100.00%> (+1.63%)	⬆️
tests/test_item.py	98.66% <100.00%> (+0.05%)	⬆️
tests/test_transition.py	95.12% <100.00%> (+3.12%)	⬆️
...-opentimelineio/opentimelineio/core/_core_utils.py	78.21% <0.00%> (+0.77%)	⬆️
src/opentimelineio/transition.cpp	100.00% <0.00%> (+18.18%)	⬆️
src/opentimelineio/mediaReference.h	100.00% <0.00%> (+30.00%)	⬆️

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[meshula]

Love it :)