Lint baseline usage feature

[awelzel]

@bbannier mentioned a stray baseline check script in Spicy. Essentially it lists all tests and reports extra/unused directories in the Baseline directory.

https://github.com/zeek/spicy/blob/main/tests/Scripts/stray_baselines.py

Chatting a bit with, maybe this could be a built-in btest lint baselines command instead.

Further, it would be cool if we could figure out concrete baseline instances, rather than just the top level directories. This likely involves some heuristics matching on <prefix>-EXEC.*btest-diff\s+<name> within the tests themselves (or using individual commands directly) given that there is no native @TEST-BASELINE-DIFF command.

Conversely, explicitly reporting baselines might be a use-case, too.

Background: https://github.com/zeek/zeek/pull/4034#discussion_r1858314653