Migrate tst_handshakeresponse to QRegularExpression

This patch updates the tst_handshakeresponse test to use
QRegularExpression in place of QRegExp which is to be considered
deprecated.

Change-Id: I98febd0565351b644dd68b98d356764bee78d7c5
Reviewed-by: Liang Qi <liang.qi@qt.io>