Fix mallocOutOfMemory test

This test was failing on gasnet+fast and darwin.  On gasnet+fast, the
virtual memory limit placed upon the test was too low to allow Chapel
to actually do anything, so it was timing out.  On darwin, the virtual
memory limit was not respected (because according to internet
sources ulimit and setrlimit do not work on OSX), so the test would
never perform as expected.  To resolve the former, I increased the
virtual memory size.  To resolve the latter, I turned off this test for
darwin.