Also, even if login.defs are not present, don't start allocating at 1, but at
SYSTEM_UID_MIN.
Fixes#9769.
The test is adjusted. Actually, it was busted before, because sysusers would
never use SYSTEM_GID_MIN, so if SYSTEM_GID_MIN was different than
SYSTEM_UID_MIN, the tests would fail. On all "normal" systems the two are
equal, so we didn't notice. Since sysusers now always uses the minimum of the
two, we only need to substitute one value.
All this test does is manipulate text files in a subdir specified with --testroot.
It can be a normal unittest without the overhead of creating a machine image.
As a bonus, also test the .standalone version.
Zbigniew Jędrzejewski-Szmek