Add regression test coverage for non-default timezone abbreviation sets.

Add regression test coverage for non-default timezone abbreviation sets.

After further reflection about the mess cleaned up in commit 39b691f25,
I decided the main bit of test coverage that was still missing was to
check that the non-default abbreviation-set files we supply are usable.
Add that.

Back-patch to supported branches, just because it seems like a good
idea to keep this all in sync.