[libc++] Fix several debug mode tests

These tests were only being run when _LIBCPP_DEBUG was defined, which
isn't the case by default when we run the test suite. In other words,
all these debug mode tests were never being run. This commit makes sure
they are run, and in some cases, extracts them into a file under test/libcxx
to separate them from the Standard tests.

Differential Revision: https://reviews.llvm.org/D88836