[libfuzzer] Disable afl tests on non-posix.

[mpividori]

Summary

AflDriver is not supported on non posix systems.

[kcc]

LGTM
put REQUIRES at the top.
And indeed, verify that the tests still run on linux after this.

llvm/trunk/lib/Fuzzer/test/afl-driver-extra-stats.test

+REQUIRES: posix
+
 ; Test that not specifying an extra stats file isn't broken.
 RUN: unset AFL_DRIVER_EXTRA_STATS_FILENAME
 RUN: AFLDriverTest
 
 ; Test that specifying an invalid extra stats file causes a crash.
 RUN: ASAN_OPTIONS= AFL_DRIVER_EXTRA_STATS_FILENAME=%T not --crash AFLDriverTest
 
 ; Test that specifying a corrupted stats file causes a crash.

llvm/trunk/lib/Fuzzer/test/afl-driver-stderr.test

+REQUIRES: posix
+
 ; Test that not specifying a stderr file isn't broken.
 RUN: unset AFL_DRIVER_STDERR_DUPLICATE_FILENAME
 RUN: AFLDriverTest
 
 ; Test that specifying an invalid file causes a crash.
 RUN: ASAN_OPTIONS= AFL_DRIVER_STDERR_DUPLICATE_FILENAME="%T" not --crash AFLDriverTest
 
 ; Test that a file is created when specified as the duplicate stderr.
 RUN: AFL_DRIVER_STDERR_DUPLICATE_FILENAME=%t AFLDriverTest
 RUN: stat %t