Turn off cygwin testing for TooManyThreads.chpl

Cygwin doesn't allow you to set the number of threads, so this test is not
useful on cygwin.
Merge pull request #738 from lydia-duncan/tooManyThreadsFix

Turn off cygwin testing for TooManyThreads.chpl

Cygwin doesn't allow you to set the number of threads, so this test is not
useful on cygwin.