QtMacExtras: Fix forcesuccess configuration

This project compiles correctly, what it's failing is the tst_Headers
postbuild test, so adding qt.qtqa-tests.insignificant instead.

Change-Id: I949f94ea9fe28d6be9175f9afcd4283da3ed13d6
Reviewed-by: Simo Fält <simo.falt@digia.com>