Skip TestThreadLocal.py on darwin temporarily for linker issue

The new static linker in Xcode 15 does not emit the necessary
symbols for file static thread local storage, causing this test
to fail when used.  The old static linker is still available
as ld-classic in Xcode 15, but it has to be invoked specially, and
the new static linker will be fixed at some point.  I may try to
add linker name and versioning information in
lldb/packages/Python/lldbsuite/test/decorators.py like we do with
the compiler / compiler_version, so it can be xfailed for known
problematic static linker name / versions, but until I get that
sorted I'm skipping this test to unblock the CI bots.