Abort testing on unsupported dialect flags

[wmaxey]

Description

closes various nvbugs where testing should not occur.

example output:

PS C:\cccl> lit -a .\build\libcudacxx-cpp20\libcudacxx\test\libcudacxx\ -j1
lit: C:\cccl\libcudacxx\test\libcudacxx\lit.cfg:45: note: Using configuration variant: libcudacxx
....
  File "C:\cccl\libcudacxx\test\utils\libcudacxx\test\config.py", line 771, in configure_default_compile_flags
    raise OSError("Configured compiler does not support flag {0}".format(self.cxx.compile_flags))
OSError: Configured compiler does not support flag ['-std=c++20']

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[miscco]