test-login: make the test non-manual

test-login.c is largely rewritten to use _cleanup_ and give more meaningful
messages (function names are used instead of creative terms like "active
session", so that when something unexpected is returned, it's much easier to
see what function is responsible).

The monitoring part is only activated if '-m' is passed on the command line.

It runs against the information from /run/systemd/ in the live system, but that
should be OK: logind/sd-login interface is supposed to be stable and both
backwards and forwards compatible.

If not running in a login session, some tests are skipped.

Those two changes together mean that it's possible to run test-login in the
test suite.

Tests for sd_pid_get_{unit,user_unit,slice} are added.