Categorize some threading libc++ test skip

[AlexGuteniev]

• atomic_ref failures are one well know annoying problem; [lib++][test] Align atomic_ref underlying variable as required llvm/llvm-project#179780 is merged.
• future failure is <future>: incorrectly used copy assignment instead of copy construction in set_value #1190 (<future>: promise default constructs the value type #2488 is the same problem described from a different angle)

[StephanTLavavej]

Thank you!! 😻 I've recorded a personal todo to remove the SKIPPED tests during the next LLVM update.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo. Please notify me if any further changes are pushed, otherwise no action is required.