Make sure lit tests pass on non-Darwin systems

lit tests always fail on non-Darwin systems because --use-isolation uses
utilities that are only available on OS X.

This patch moves the test to an Darwin-only test case, so that unit tests can
be ran by Linux buildbots.


git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@216290 91177308-0d34-0410-b5e6-96231b3b80d8