include <QTest> rather than <QtTest>

Not including the entire QtTest module and its dependencies should
reduce test compilation time.

GIT_SILENT
1 job for !222 with dont-include-qttest in 34 minutes and 7 seconds (queued for 11 minutes and 50 seconds)
detached
Status Job ID Name Coverage
  Build
passed #91730
linux kf5-qt5 qt5.15

00:34:07

46.0%