Add a unit test for testing Qt in its install location.

The Qt CI system runs the unit tests after installation, but
with the qmake in the build directory. This means that the
installed content is not unit tested. Add an additional cmake
unit test to test the files in the install location.

The new test is marked insignificant for now until the true
effect on the CI system is known.

Task-number: QTBUG-27315

Change-Id: If9f12e88cfc741946cfabc25dbf789a11a2af4b8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>