build: Add installed-tests support

This means the tests are installed on the system, and can be run using
   gnome-desktop-testing-runner geocode-glib-1.0

This means they will be run as integration tests on systems like
gnome-continuous, which may help to catch bugs caused by integration
changes with other modules.

The tests themselves have not actually changed.

https://bugzilla.gnome.org/show_bug.cgi?id=772928