coverage: Add UI tests for values accepted by `-Cinstrument-coverage`
I wanted to clean up the code in `parse_instrument_coverage`, but it occurred to me that we currently don't have any UI tests for the various stable and unstable values supported by this flag.
---
Normally it might be overkill to individually test all the different variants of `on`/`off`, but in this case the parsing of those values is mixed in with some other custom code, so I think it's worthwhile being thorough.
46d7038b03