Don't build empty tests.

Don't build empty tests.

These tests need the xmlpatterns module.  If that module is not in the
Qt build, then avoid building the test rather than building an empty
test.

Change-Id: Id0f72cfc9f818096361ecbc439196d2d5c64322e
Reviewed-on: http://codereview.qt-project.org/6487
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>