validate:launcher: skip setting up test suite for --medias-paths

in validate.py, some mixer test generators are being added by default.
When passing --media-paths, i would not want to test these.
So instead of setting up the validate test suite, just call tester.register_defaults().

https://bugzilla.gnome.org/show_bug.cgi?id=752518

validate/launcher/baseclasses.py

@@ -1137,7 +1137,10 @@ class _TestsLauncher(Loggable):
                         tester.name not in wanted_test_manager:
                     continue
 
-                if testsuite.setup_tests(tester, self.options):
+                if self.options.paths:
+                    tester.register_defaults()
+                    loaded = True
+                elif testsuite.setup_tests(tester, self.options):
                     loaded = True
 
             if not loaded: