CI: Run "as-installed" tests with Meson, not just Autotools

Prerequisite for !378 (merged).

• meson: Allow sudo meson install, which doesn't set DESTDIR

  This is generally not recommended (dbus should usually be installed from the OS distributor's packages, which should install into a staging directory using DESTDIR), but we'll want to use it in Gitlab-CI.

• CI: Run "as-installed" tests with Meson, not just Autotools

  Otherwise, we'd lose this test coverage when we remove Autotools.

  Resolves: #436 (closed)

• CI: Install systemd system and user units into our ${prefix}

  This keeps everything we install from CI separate from a possible OS-level installation in /usr.

  Resolves: #470 (closed)