Mark tst_qcamerabackend test as insignificant on Mac OS X

It has failed 6 times during qt5.git stable integrations since September 2013

See also QTBUG-36040

Change-Id: I6abd5b2723d2346a4831ab28fa040c14615148b0
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>