Settings: Support LIBOPENSHOT_DEBUG envvar

[ferdnyc]

This was originally part of my PR #593, but that was too ambitious and is stuck on hold for the moment. So, I broke this useful feature out into its own PR. With this change, setting LIBOPENSHOT_DEBUG to any value in the process environment directs the ZeroMQ debug logging stream to stderr (in addition to any other file/subscriber destinations).

[codecov]

Codecov Report

Merging #667 (97c2530) into develop (a00bbb7) will decrease coverage by 0.45%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #667      +/-   ##
===========================================
- Coverage    52.42%   51.97%   -0.46%     
===========================================
  Files          151      151              
  Lines        12346    12862     +516     
===========================================
+ Hits          6473     6685     +212     
- Misses        5873     6177     +304     

Impacted Files	Coverage Δ
src/Settings.h	50.00% <ø> (ø)
src/Settings.cpp	100.00% <100.00%> (ø)
tests/Settings.cpp	100.00% <100.00%> (ø)
tests/Point.cpp	100.00% <0.00%> (ø)
src/effects/Tracker.cpp	0.00% <0.00%> (ø)
src/CVObjectDetection.cpp	0.00% <0.00%> (ø)
src/ClipProcessingJobs.cpp	0.00% <0.00%> (ø)
src/effects/Stabilizer.cpp	0.00% <0.00%> (ø)
src/effects/ObjectDetection.cpp	0.00% <0.00%> (ø)
src/CVStabilization.cpp	89.51% <0.00%> (+1.33%)	⬆️
... and 2 more

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[ferdnyc]

I also added a unit test for the feature (which involved working out how to have CTest configure the environment for certain unit tests), and added a VERBOSE_TESTS option to the CMake configuration. It defaults OFF, but if enabled on the command line will run ctest with the -VV argument, causing it to output detailed setup/execution information as each test is run. (It also shows the stdout/stderr of the tests themselves, which is normally suppressed.)

[ferdnyc]

With testing in place, I'm going to merge this. It's handy to have, and shouldn't affect anything else unless it's used.