Turn off mallocOutOfMemory for cygwin

Cygwin also fails to define a useful ulimit function, so any attempts to limit
the memory of this test will not work.  Even though the test will still run and
complete with a higher number of iterations, I think we should treat it the way
we treat darwin and skip it.
Merge pull request #737 from lydia-duncan/fixMallocCygwin

Turn off mallocOutOfMemory for cygwin [reviewed by hilde]

Cygwin also fails to define a useful ulimit function, so any attempts to limit
the memory of this test will not work. Even though the test will still run and
complete with a higher number of iterations, I think we should treat it the way
we treat darwin and skip it.  While I was there, I altered the valgrind skipif to
skip the valgrind exec-only case (since that seemed to be the point).