qt-5: Don't build tests

Building the tests (which as far as I can tell we don't even run)
makes the build take > 19 GiB of disk space, which is a bit
excessive. Without the tests, it takes 2.6 GiB.
qt-5: Prevent some unnecessary runtime dependencies

The file $out/mkspecs/qconfig.pri contains a list of all include /
library paths used during the build, including build-time-only
packages like GDB and Ruby. So get rid of those.