QtQA: Added configurable parameter for test dir

By using qt.tests.dir parameter one can define where the
build will run the tests. Default is the submodule root, as it is
now. Affects only qt5 builds.

Change-Id: I7c7f5610f092f4510a0880d3df9af70bfe814b93
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Janne Anttila <janne.anttila@digia.com>