[libFuzzer] XFAIL fuzzer-oom.test on Darwin.

The test fails on Darwin because Fuzzer::DeathCallback (which calls
DumpCurrentUnit("crash-")) is called before DumpCurrentUnit("oom-") is
called in Fuzzer::RssLimitCallback. DeathCallback is transitively called
from __sanitizer_print_memory_profile.

This should fix the fuzzer bot that has been failing for a while:

http://lab.llvm.org:8080/green/job/libFuzzer/

llvm-svn: 300127