Fixed up intermittently failing tests to skip on Linux.

Also added 'import sys' on some tests that are using non-standard
unittest2.skipUnless blocks with code that is intended to do things
that we have more specializes @* attributes for.  These skip
conditions were failing to execute due to missing import, causing
darwin-only tests to run on Linux regardless.  Will file a bug for
that separately.

llvm-svn: 204747