build: add --enable-installed-tests parameter

See https://live.gnome.org/GnomeGoals/InstalledTests for more information.
Those tests will also be executed on http://build.gnome.org. For now we run
all our suite as one big test

It's still possible to run `make check` with locally uninstalled tests.

https://bugzilla.gnome.org/show_bug.cgi?id=734370