[Variant] Add testing for invalid variants (fuzz testing??)

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

• @scovich added some great changes in [VARIANT] Support both fallible and infallible access to variants #7807 to improve the validation APIs in variant.

However, we don't really have much coverage of the validation code (specifically validation on input errors).

Describe the solution you'd like

I would like additional coverage of the variant error cases

Describe alternatives you've considered

I do think trying to cook up examples of all possible malformed variants is probably not a great use of time, but writing a fuzz tester might be

Maybe we could create a fuzz tester that created a random variant using the builder, and then randomly changed a few bytes around. -- Then if validation succeeded we would walk over the entire variant and verify that the infallable APIs didn't panic

I think fuzzing would help a lot, especially with AFL

Originally posted by @friendlymatthew in #7807 (comment)
Additional context

[alamb]

label_issue.py automatically added labels {'parquet'} from #7807