Tests for help messaging from AthConfigFlags
As requested by @fwinkl I have added some tests for the help messaging functionality, that shows:
- Navigation through the flag categories
- 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.