[lldb] [test] Use skipUnlessDarwin for tests specific to Darwin

Use skipUnlessDarwin decorator for tests that are specific to Darwin,
instead of skipIf... for all other platforms.  This should make it clear
that these tests are not supposed to work elsewhere.  It will also make
these tests stop repeatedly popping up while I look for tests that could
be fixed on the platform in question.

Differential Revision: https://reviews.llvm.org/D91003