[pitivi] Calm down test_system_gnome

[Jean-François Fortin Tam <jfft src gnome org>]

commit 2b140a5fe01c1989761374f8ff1e0914828a0a0b
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Tue May 21 19:19:45 2013 +0200

    Calm down test_system_gnome

 tests/test_system_gnome.py |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/tests/test_system_gnome.py b/tests/test_system_gnome.py
index a1c5712..e1ef82f 100644
--- a/tests/test_system_gnome.py
+++ b/tests/test_system_gnome.py
@@ -9,7 +9,10 @@ class TestGnomeSystem(TestCase):
         self.system = getSystem()
 
     def testPowerInhibition(self):
-        self.assertTrue(isinstance(self.system, GnomeSystem))
+        if not isinstance(self.system, GnomeSystem):
+            # We can only test this on a Gnome system.
+            return
+
         #check that no other programs are inhibiting, otherwise the
         #test is compromised
         self.assertTrue(not self.system.session_iface.IsInhibited(