Ideally in the future we change more of our unit tests to
support running installed; we've tried this in the past with
https://wiki.gnome.org/Initiatives/GnomeGoals/InstalledTests
I'd like to pick that back up again. This takes a step
towards that by having our Rust tests.
To make this even easier, add a `tests/run-installed`
which runs the installed tests (uninstalled, confusingly
but conveniently for now).
Colin Walters