Support for multiple btest baselines

[vpax]

I'd like to be able to use btest's --alternative option as a way to conduct testing of Zeek options that alter Zeek's behavior on only a small number of tests, and for which I'd like to make sure that on the bulk of the tests the options don't result in any changes. While there's already a notion in btest of alternative baselines (via BTEST_TEST_BASE), that changes the entire baseline. What I want instead is essentially a series of baseline directories that are searched first-to-last. With that, then I can put updates for the given Zeek option in Baseline.my_alternative/ and confine what goes in there to just the differences, still picking up the bulk of the baseline from the default Baseline/. I'd also want btest -u to update into the alternative Baseline rather than the original.

[ckreibich]

Vern, this is primarily a btest feature request, so we could move it over to github.com/zeek/btest — yes? (Happy to do that, just wanted to confirm.)
The following might be a workaround in the interim: you can make the (few) tests whose behavior differs when the option is set conditional on that setting. For example, if your option is going to be ZEEK_SPECIAL_FLAG=vern, set that in an alternative, and say @TEST-REQUIRES: test "${ZEEK_SPECIAL_FLAG}" == "vern" and the test will simply be skipped when that requirement isn't fulfilled. The other tests will still run with the option, but for those the expectation is that their behavior remains unchanged.

[vpax]

@ckreibich happy to move this ot github.com/zeek/btest, thanks for pointing that out. (Still getting acquainted with all of the angles of our ecosystem! :-) I'll try out what you sketch flag-wise to see if it does suffices in the interim.

[ckreibich]

Still getting acquainted with all of the angles of our ecosystem!

😄 — it's bigger on the inside, isn't it!

[rsmmr]

I'll give this a try.

[rsmmr]

This has landed.