[pitivi] tests: Fix mainloop timeout detection

[Alexandru Băluț <alexbalut src gnome org>]

commit 6756a1d58ca58a5525d72d02668ddde1fb718917
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Wed Apr 27 08:22:13 2016 +0200

    tests: Fix mainloop timeout detection
    
    Differential Revision: https://phabricator.freedesktop.org/D970

 tests/common.py |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/tests/common.py b/tests/common.py
index 48ad095..12d13a4 100644
--- a/tests/common.py
+++ b/tests/common.py
@@ -94,6 +94,7 @@ def create_main_loop():
     timed_out = False
 
     def quit_cb(unused):
+        nonlocal timed_out
         timed_out = True
         mainloop.quit()