tests/inst: Make nondestructive tests runnable as unit tests

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).

tests/run-installed

@@ -0,0 +1,6 @@
+#!/bin/bash
+# Run this script after e.g. `sudo make install` in a development container/host
+# to run tests which operate on the installed binaries.
+set -xeuo pipefail
+dn=$(dirname $0)
+(cd ${dn}/../tests/inst && cargo run --release)