Remove insignificant_test for tst_qwindow on Mac OS X

tst_QWindow::positioning() still fails on Mac OS X, and it does so in
several places. Skip this test for now as it causes isActive() to fail
as well. With positioning() QSKIP()ed, all the other test functions
pass:

Totals: 19 passed, 0 failed, 1 skipped
********* Finished testing of tst_QWindow *********

Task-number: QTBUG-23059
Change-Id: I58d036120c0121f515813cd20955ab3b82f81fe1
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>