It has been completely untested for a while and saw some breakage.
So let's add a selftest for it. ('-vs' when running tests)
Change-Id: Ibfb5ac0a2d741de7c3f519d91202d4977996045e
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
