sanitize_commit: skip "hygiene" checks on files in tests/.../testdata/ directories

Various of the inanity bot's checks are directed towards conforming with best practices.
Not infrequently, test data files violate these best practices (e.g. no newline at end of file) for good reasons; in any case, it's test data, it typically needs to be what it is, and its defects are apt to be necessary for the data to stress what's being tested.
It is tiresome to have the 'bot grumble about such inane matters.

Likewise, tests/.../testcases/ are used by fuzzers, which tend to like nonsensical files as test-cases, to which we don't want to apply these hygiene checks.