Add more arg testing for asn1_codec.py and common.py
and exclude a couple of lines from coverage metrics since they're
only there to try to be helpful if there is a faulty installation
or if the code changes and impossible things become possible.
Coverage increased (+0.7%) to 96.736% when pulling f5c5823cafa5b2751a1874bf449838dfdd40b558 on awwad:additional_coverage into 709e780f6c0e322c798584fe808075870051d441 on uptane:develop.
Add more arg testing for asn1_codec.py and common.py and exclude a couple of lines from coverage metrics since they're only there to try to be helpful if there is a faulty installation or if the code changes and impossible things become possible.