tst_headers: explain the _q_ private slots check
tst_headers: explain the _q_ private slots check

Make the failure message more helpful.

Change-Id: Idd7e66c3fc635a1e9304ec84aa0b84386561017c
Sanity-Review: Qt Sanity Bot <5581206bb7e0307f0d99eb71898ae6b694149ca5@ovi.com>
Reviewed-by: Jason McDonald <40c1146f72b16acca7bc08c70880d435c5d12691@nokia.com>
Reviewed-by: Sergio Ahumada <27ca633ce817d1c3858ceeac3c99febb78411853@nokia.com>

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