[libc++][test] Adds backdeployment shorthands. (#78204)

Some changes in libc++ affect the dylib. These changes are not present
on systems that use the system dylib. Currently that are the Apple
backdeployment targets. Figuring out which MacOS versions to target is
not trivial for non-Apple engineers. These shorthands make it easier to
select the proper feature make a test UNSUPPORTED or XFAIL.

During the design discussion with Louis we considered whether or not to
add preprocessor definitions to allow partial disabling of a test. This
would be useful when an existing feature is changed by modifying the
dylib. In the end we decided not to add this feature to avoid additional
complexity in the tests. Instead the test will be disabled for that
target.