Disable tests on failing ANGLE config

Task-number: QTBUG-40658
Change-Id: I9a00083213de4d31216813351a406ec460d146fa
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Temporarily mark grab()/item.layer tests insignificant under ANGLE

This is to pass CI while the issue is being fixed.

Task-number: QTBUG-40649
Change-Id: I9b095ce0ce0a3c51956ab81d09975b6cc6ce3592
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>