Tests for help messaging from AthConfigFlags

Will Buttinger requested to merge will/athena:AthConfigFlags_furtherTests into 23.0

As requested by @fwinkl I have added some tests for the help messaging functionality, that shows:

  1. Navigation through the flag categories
  2. Overriding displayed default value for flags (useful for seeing the effect of a flag override)

There was a minor improvement to the AthConfigFlags class to allow user to specify constructor arguments to ArgumentParser ... in developing the tests I added this but in the end it wasn't used, but I thought it would still be helpful to leave this in for possible future work.

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