test-login: skip consistency checks when logind is not active

There are two ways in swich sd_login_* functions acquire data:
some are derived from the cgroup path, but others use the data serialized
by logind.

When the tests are executed under Fedora's mock, without systemd-spawn
but instead in a traditional chroot, test-login gets confused:
the "outside" cgroup path is visible, so sd_pid_get_unit() and
sd_pid_get_session() work, but sd_session_is_active() and other functions
that need logind data fail.

Such a buildroot setup is fairly bad, but it can be encountered in the wild, so
let's just skip the tests in that case.

/* Information printed is from the live system */
sd_pid_get_unit(0, …) → "session-237.scope"
sd_pid_get_user_unit(0, …) → "n/a"
sd_pid_get_slice(0, …) → "user-1000.slice"
sd_pid_get_session(0, …) → "237"
sd_pid_get_owner_uid(0, …) → 1000
sd_pid_get_cgroup(0, …) → "/user.slice/user-1000.slice/session-237.scope"
sd_uid_get_display(1000, …) → "(null)"
sd_uid_get_sessions(1000, …) → [0] ""
sd_uid_get_seats(1000, …) → [0] ""
Assertion 'r >= 0' failed at src/libsystemd/sd-login/test-login.c:104, function test_login(). Aborting.

(cherry picked from commit ac564463)