Fix running tst_qmessagehandler when QT_MESSAGE_PATTERN is set

This test fails if the environment has the variable set.

Change-Id: Ibd54ff3e6e22a885341898889088ac56e84282b1
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>