[lldb/test] Skip TestCppIncompleteTypeMembers.py on Darwin (NFC)

This skips `TestCppIncompleteTypeMembers.py` on Darwin platforms since
it requires `-flimit-debug-info` which is not supported.

This should fix the Green Dragon bot test run:

https://green.lab.llvm.org/green/job/lldb-cmake/43678



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>