[libcxx][test] Attempt to make debug mode tests more bulletproof

The problem with debug mode tests is that it isn't known which particular
_LIBCPP_ASSERT causes the test to exit, and as shown by
https://reviews.llvm.org/D100029 and 2908eb20 it might be not the
expected one.

The patch adds TEST_LIBCPP_ASSERT_FAILURE macro that allows checking
_LIBCPP_ASSERT message to ensure we caught an expected failure.

Reviewed By: Quuxplusone, ldionne

Differential Revision: https://reviews.llvm.org/D100595