Bug 716140 - Mark test_preferences.xul as sometimes asserting once.

toolkit/content/tests/chrome/test_preferences.xul

@@ -14,6 +14,8 @@
   <![CDATA[
     if (navigator.platform.startsWith("Mac")) {
       SimpleTest.expectAssertions(4);
+    } else {
+      SimpleTest.expectAssertions(0,1);
     }
     SimpleTest.waitForExplicitFinish();