[libc++] Test suite adjustments on macOS (#95835)

This patch makes a few adjustments to the way we run the tests in the
Apple configuration on macOS:

First, we stop using DYLD_LIBRARY_PATH. Using that environment variable
leads to libc++.dylib being replaced by the just-built one for the whole
process, and that assumes compatibility between the system-provided
dylib and the just-built one. Unfortunately, that is not the case
anymore due to typed allocation, which is only available in the system
one. Instead, we want to layer the just-built libc++ on top of the
system-provided one, which seems to be what happens when we set a rpath

Second, add a missing XFAIL for a std::print test that didn't work as
expected when building with availability annotations enabled. When we
enable these annotations, std::print falls back to a non-unicode and
non-terminal output, which breaks the test.

GitOrigin-RevId: 2cf2f1b2b6219b606faeade2342c3d2288658ab0
4 files changed