tests: Check return value of clutter_init_with_args instead of error

Some of the tests were ignoring the return value of
clutter_init_with_args and instead they would recognise an error by
seeing whether the GError parameter was set. This patch changes it to
check the return value so that it won't give a warning now that
G_GNUC_WARN_UNUSED_RESULT is on that function.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2574

tests/interactive/test-drag.c

@@ -156,12 +156,11 @@ test_drag_main (int argc, char *argv[])
   GError *error;
 
   error = NULL;
-  clutter_init_with_args (&argc, &argv,
-                          "test-drag",
-                          entries,
-                          NULL,
-                          &error);
-  if (error != NULL)
+  if (clutter_init_with_args (&argc, &argv,
+                              "test-drag",
+                              entries,
+                              NULL,
+                              &error) != CLUTTER_INIT_SUCCESS)
     {
       g_print ("Unable to run test-drag: %s\n", error->message);
       g_error_free (error);