[libc++][libc++abi] Fix or suppress failing tests in single-threaded

builds.

Fix a libc++abi test that was incorrectly checking for threading
primitives even when threading was disabled.

Additionally, temporarily XFAIL some module tests that fail because
the <atomic> header is unsupported but still built as a part of the
std module.

To properly address this libc++ would either need to produce a different
module.modulemap for single-threaded configurations, or it would need
to make the <atomic> header not hard-error and instead be empty
for single-threaded configurations