Running the tests without availability enabled doesn't really make sense:
availability annotations allow catching errors at compile-time instead
of link-time. Running the tests without availability enabled allows
confirming that a test breaks at link-time under some configuration,
but it is more useful to instead check that it should fail at compile-time.
Always enabling availability in the lit test suite will greatly simplify
XFAILs and troubleshooting of failing tests, which is currently a giant
pain because we have these two levels of possible failure: link-time and