tests:glcolorconvert: Fix caps memory leak

Output caps being got from video info is not getting freed

https://bugzilla.gnome.org/show_bug.cgi?id=757929

tests/check/libs/gstglcolorconvert.c

@@ -204,6 +204,7 @@ check_conversion (TestFrame * frames, guint size)
         }
       }
 
+      gst_caps_unref (out_caps);
       gst_video_frame_unmap (&out_frame);
       gst_buffer_unref (outbuf);
     }