build: make reftests work without installed tests
build: make reftests work without installed tests

The libreftest.so module needs to be built in all cases.
Without installed tests, it needs to not be installed though.

https://bugzilla.gnome.org/show_bug.cgi?id=735401

Adamovskiy/gtk,davidgumberg/gtk,Lyude/gtk-,ahodesuka/gtk,chergert/gtk,ahodesuka/gtk,jigpu/gtk,jessevdk/gtk,jessevdk/gtk,grubersjoe/adwaita,alexlarsson/gtk,Adamovskiy/gtk,Lyude/gtk-,chergert/gtk,grubersjoe/adwaita,Lyude/gtk-,grubersjoe/adwaita,alexlarsson/gtk,davidgumberg/gtk,grubersjoe/adwaita,chergert/gtk,chergert/gtk,grubersjoe/adwaita,jadahl/gtk,ahodesuka/gtk,davidgumberg/gtk,Adamovskiy/gtk,Lyude/gtk-,jessevdk/gtk,jadahl/gtk,chergert/gtk,jessevdk/gtk,alexlarsson/gtk,Lyude/gtk-,chergert/gtk,ahodesuka/gtk,jigpu/gtk,alexlarsson/gtk,jessevdk/gtk,jigpu/gtk,Adamovskiy/gtk,jadahl/gtk,Lyude/gtk-,davidgumberg/gtk,ahodesuka/gtk,Adamovskiy/gtk,jadahl/gtk,jessevdk/gtk,jadahl/gtk,jigpu/gtk,jigpu/gtk,chergert/gtk,Lyude/gtk-,grubersjoe/adwaita,ahodesuka/gtk,ahodesuka/gtk,davidgumberg/gtk,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,jigpu/gtk,grubersjoe/adwaita,jigpu/gtk,Adamovskiy/gtk,jessevdk/gtk,jadahl/gtk,davidgumberg/gtk,Adamovskiy/gtk,davidgumberg/gtk,ahodesuka/gtk,davidgumberg/gtk,Adamovskiy/gtk,grubersjoe/adwaita,Lyude/gtk-,jadahl/gtk,alexlarsson/gtk,jadahl/gtk,chergert/gtk,alexlarsson/gtk