Respect Output Format In Packager Test

Make sure to use the output format when given when setting up our
tests. Not doing so results in text always being set to "vtt" when
the output format is "mp4".

Change-Id: I11c5f861091598a67fc76dc19b1b16a9a773a2e0

packager/app/test/packager_test.py

@@ -92,11 +92,11 @@ def _UpdateMpdTimes(mpd_filepath):
 
 
 def GetExtension(stream_descriptor, output_format):
+  if output_format:
+    return output_format
   # TODO(rkuroiwa): Support ttml.
   if stream_descriptor == 'text':
     return 'vtt'
-  if output_format:
-    return output_format
   # Default to mp4.
   return 'mp4'