tests: Don't assume uid == primary gid

tests: Don't assume uid == primary gid

Nothing guarantees that each user has a group containing only
themselves. Even if they do, nothing guarantees that its group ID
equals the user ID, particularly if another user earlier in the same
range was created without a corresponding group or vice versa.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Forwarded: https://github.com/ostreedev/ostree/pull/1390
Applied-upstream: 2018.1, commit:https://github.com/ostreedev/ostree/commit/f63e62fbd272edbdd9695a10d841fc6b5eb69df8

Gbp-Pq: Topic 2018.1
Gbp-Pq: Name tests-Don-t-assume-uid-primary-gid.patch