selftest: sync: stress test for parallelism

This test is based on the libsync test suite from Android.
This commit includes a stress test that invokes operations
in parallel.

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

@@ -54,4 +54,7 @@ int test_fence_merge_same_fence(void);
 /* Fence wait tests */
 int test_fence_multi_timeline_wait(void);
 
+/* Stress test - parallelism */
+int test_stress_two_threads_shared_timeline(void);
+
 #endif