Add bug number for unstable test.

Task-number: QTBUG-25483
Change-Id: Ic2778a5b23950a698eb678938a38afe7317421d1
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

tests/auto/linguist/lupdate/lupdate.pro

@@ -3,7 +3,7 @@ QT += testlib
 
 # June 2011: Marked as unstable due to tst_lupdate::good(heuristics) failing
 # due to changes in qtbase/mkspecs/features/qt_config.prf.
-CONFIG+=insignificant_test
+CONFIG+=insignificant_test  # QTBUG-25483
 
 TARGET = tst_lupdate