[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

llvm/lib/Fuzzer/test/fuzzer-oom.test

@@ -1,3 +1,4 @@
+XFAIL: darwin
 RUN: not LLVMFuzzer-OutOfMemoryTest -rss_limit_mb=300 2>&1 | FileCheck %s
 
 CHECK: ERROR: libFuzzer: out-of-memory (used: {{.*}}; limit: 300Mb)