[Fuzzing] XFAIL Fuzzing tests for macOS.

A change to the LibFuzzer runtime means the old coverage instrumentation
isn't supported anymore. Until we have a fix ready
(rdar://problem/49853280) we should just XFAIL the tests.

rdar://problem/48621055

Author: danliew-apple

test/Fuzzing/fuzzer_test.swift

@@ -3,6 +3,8 @@
 // REQUIRES: CPU=x86_64
 // REQUIRES: executable_test
 // REQUIRES: fuzzer_runtime
+// TODO(dliew): Re-enable for macosx (rdar://problem/49853280)
+// XFAIL: OS=macosx
 // XFAIL: OS=ios
 // XFAIL: OS=tvos
 // XFAIL: OS=watchos

test/Fuzzing/fuzzer_test_simpler.swift

@@ -4,6 +4,8 @@
 // REQUIRES: CPU=x86_64
 // REQUIRES: executable_test
 // REQUIRES: fuzzer_runtime
+// TODO(dliew): Re-enable for macosx (rdar://problem/49853280)
+// XFAIL: OS=macosx
 // XFAIL: OS=ios
 // XFAIL: OS=tvos
 // XFAIL: OS=watchos