opencv: fix qt selection when highgui is enable

The OpenCV highgui module links against QtGui and QtTest.
The latter is built by default but was never installed.
Thanks to the previous patch on Qt, this can be rectified.

So, update the OpenCV Config.in with the missing Qt module selection.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>