qt: add an option to install the test module

When building Qt in a minimal configuration, a couple of Qt modules
(QtNetwork, QtSql, QtTest and QtXml) are built in addition to QtCore,
so they are installed in the sysroot but they are not copied into the
target tree.

Among these modules that could be built but optionally installed, only
QtTest had no option to allow being installed in the target fs.

Thus, programs linking against QtTest would successfully build, but
would miserably fail at runtime due to this missing dependency.

So far, the only package triggering this issue is OpenCV (the highgui
module built with Qt support).

A followup patch will add this missing dependency in OpenCV using this
new option.

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>