Fix tst_qquickpositioners on OS X

Done-with: Jan Arve Sæther
Change-Id: Iff66f62167a5977354319aac311d2cb81d9f3842
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>
Make test more robust against ignored children.

With the upcoming change that changes the semantics of
Accessible.ignored, some assumptions in the test does not hold anymore.

Change-Id: I9b34f1ecc61fc0d4e8abe5a5505a5bcff44501d4
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>