doc: Add documentation to pthread_mutex_perf application

Signed-off-by: Alan C. Assis <acassis@gmail.com>

Author: acassis

Documentation/applications/testing/pthread_mutex_perf/index.rst

@@ -0,0 +1,15 @@
+=======
+Testing
+=======
+
+The ``pthread_mutex_perf`` is a simple performance test for validate if some
+specific kernel modification impacted the pthread_mutex_trylock().
+
+Basically the test will run a busy-wait 1000000 trying to acquire a mutex
+that is already locked. Then it will calculated the total time and the
+average time repeating this cycle 10 times.
+
+This test is not intented to confirm that pthread mutex is working, for that
+is better to use ostest. Its goal is to verify impact in the kernel that
+could be considered a regression.
+