Mark tests tst_qquickloader as insignificant due to random failures

Task-number: QTBUG-30721

Change-Id: I540848b1e7c9992e89b6df2417c37ce5e81423c8
Reviewed-by: Alan Alpert (Personal) <416365416c@gmail.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>