This patch allows skipping a test based on a default setting, which is useful when running the testsuite in different "modes" based on a default setting. This is a feature I need for the Swift testsuite, but I think it's generally useful.
See also https://reviews.llvm.org/D72662 for context.
This won't trip if the tests doesn't run. If you make the assert trip and XFAIL the test it should catch it.