tests: Update GMutexLocker tests

Spawn a thread and assert that the mutex actually got locked and then
unlocked again, same as we do in GRecMutexLocker tests.

glib/tests/autoptr.c

@@ -329,10 +329,30 @@ test_g_mutex (void)
   g_mutex_init (&val);
 }
 
+/* Thread function to check that a mutex given in @data is locked */
+static gpointer
+mutex_locked_thread (gpointer data)
+{
+  GMutex *mutex = (GMutex *) data;
+  g_assert_false (g_mutex_trylock (mutex));
+  return NULL;
+}
+
+/* Thread function to check that a mutex given in @data is unlocked */
+static gpointer
+mutex_unlocked_thread (gpointer data)
+{
+  GMutex *mutex = (GMutex *) data;
+  g_assert_true (g_mutex_trylock (mutex));
+  g_mutex_unlock (mutex);
+  return NULL;
+}
+
 static void
 test_g_mutex_locker (void)
 {
   GMutex mutex;
+  GThread *thread;
 
   g_mutex_init (&mutex);
 
@@ -340,8 +360,16 @@ test_g_mutex_locker (void)
     {
       g_autoptr(GMutexLocker) val = g_mutex_locker_new (&mutex);
       
-      g_assert (val != NULL);
+      g_assert_nonnull (val);
+
+      /* Verify that the mutex is actually locked */
+      thread = g_thread_new ("mutex locked", mutex_locked_thread, &mutex);
+      g_thread_join (thread);
     }
+
+    /* Verify that the mutex is unlocked again */
+    thread = g_thread_new ("mutex unlocked", mutex_unlocked_thread, &mutex);
+    g_thread_join (thread);
 }
 
 /* Thread function to check that a recursive mutex given in @data is locked */