tst_QWidget: include QTest, not QtTest

... which is the module-wide include, bringing in the entire QtCore.

Change-Id: I5cd872efa4562917fc4e3850809cb7595710c43a
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>