Update a few tests that may change the platform to save & restore

the platform in the setUp/tearDown methods.  I want to migrate the
re-instatement of the correct plaform to the setUp base method but
haven't had time to look at that yet, so I want to land this handful
of fixes until I get to it.

Differential revision: https://reviews.llvm.org/D66331

llvm-svn: 369484