tst_qquickflickable: unskip rebound on OS X

Out of 200 runs on 10.9 + stable branch, I have 0 failures.

Task-number: QTBUG-26696
Change-Id: I84de15b68dfff2cb5f141c5f2a783154db16b77e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>