qtmod_test.pl: disable test backtraces by default on mac
qtmod_test.pl: disable test backtraces by default on mac

Core dumps of crashing Qt tests on mac are unacceptably slow (in the
order of tens of minutes).  Disable them by default.

Change-Id: Ia66be96d3866a6c8db316313990c7b25aec37c3c
Sanity-Review: Qt Sanity Bot <5581206bb7e0307f0d99eb71898ae6b694149ca5@ovi.com>
Reviewed-by: Jyri Tahtela <f4a6700497ee4baf1b609e02c6d9f260ad39b761@nokia.com>

tsarajar/qtqa,tsarajar/qtqa,tsarajar/qtqa,tsarajar/qtqa