Changed this test to use rmutex since this has a little bit better code coverage.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403346

dlib/test/threads.cpp

@@ -30,8 +30,8 @@ namespace
         {}
 
         thread_specific_data<int> tsd;
-        mutex cm;
-        signaler sm;
+        rmutex cm;
+        rsignaler sm;
         int count;
         bool failure;