[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.

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

 RUN: [[ $(grep "peak_rss_mb\|slowest_unit_time_sec" %t | wc -l) -eq 2 ]]
 
 ; Test that peak_rss_mb and slowest_unit_time_in_secs are only updated when necessary.
 ; Check that both lines have 9999 since there's no way we have exceeded that
 ; amount of time or virtual memory.
 RUN: printf "peak_rss_mb : 9999\nslowest_unit_time_sec: 9999\n" > %t
 RUN: AFL_DRIVER_EXTRA_STATS_FILENAME=%t AFLDriverTest
 RUN: [[ $(grep "9999" %t | wc -l) -eq 2 ]]
+
+REQUIRES: posix

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

 ; 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
+
+REQUIRES: posix