tst_QDateTime::operator_insert_extract() was setting the time-zone and
taking care to restore it at the end of the test; however, if the test
were to fail, the restore would be skipped. Package the zone-setting
and restore in a class instance, so that premature return can't bypass
the restore.
Change-Id: I3df63260da17e481ef4d0d107d9f0fdea3e147e7
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>