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.
This extends the "uid:gid" syntax for "u" lines so that a group
name can be given instead of a GID. This requires that the group
is either queued for creation by sysusers, or it is already defined
on the system.
Closes#14340