Allow mallOutOfMemory to complete with gasnet+fast

This test was setting virtual memory low in an attempt to get the test to
complete quickly.  Gasnet+fast seems to need more memory to work at all, though,
so bump the limit.
Turn off mallocOutOfMemory for darwin, as it does not behave appropriately

This test depends on the ability to limit the amount of virtual memory so that
the process will attempt to allocate too much and exit with an error message.
Unfortunately, various internet sources tell me that Darwin's ways to limit
virtual memory do not work.  Since the alternative is a test that would stomp
all over everything if left unchecked and that will always time out in our
testing system, I'm turning it off for darwin.
Merge pull request #702 from lydia-duncan/fixOutOfMemory

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.