selftest: sync: stress consumer/producer test

This test is based on the libsync test suite from Android.
This commit includes a stress test that replicates a
consumer/producer pattern.

Signed-off-by: Emilio López <emilio.lopez@collabora.co.uk>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/sync/synctest.h

@@ -57,4 +57,7 @@ int test_fence_multi_timeline_wait(void);
 /* Stress test - parallelism */
 int test_stress_two_threads_shared_timeline(void);
 
+/* Stress test - consumer */
+int test_consumer_stress_multi_producer_single_consumer(void);
+
 #endif