Let's make this behave more like all the rest of the meson stuff.
This also is the first step to making it a bit more flexible so we
can define integration tests in different ways as will be seen in
the next commits.
- use test_append_files() to install additional commands
- drop use of expect
- include assert.sh and use assertions at several places
- use timeout command at several places
- always use logind-test-user
- etc