On systems with remote groups, the primary user group may be remote and may not exist on
the local system (i.e., it might just be a number). On the CLI, it looks like this:
```console
> touch foo
> l foo
-rw-r--r-- 1 gamblin2 57095 0 Dec 29 22:24 foo
> chmod 2000 foo
chmod: changing permissions of 'foo': Operation not permitted
```
Here, the local machine doesn't know about per-user groups, so they appear as gids in
`ls` output. `57095` is also `gamblin2`'s uid, but the local machine doesn't know that
`gamblin2` is in the `57095` group.
Unfortunately, it seems that Python's `os.chmod()` just fails silently, setting
permissions to `0o0000` instead of `0o2000`. We can avoid this by ensuring that the file
has a group the user is known to be a member of.
- [x] Add `ensure_known_group()` in the permissions tests.
- [x] Call `ensure_known_group()` on tempfile in `test_chmod_real_entries_ignores_suid_sgid`.
06312ddf18