Skip chown tests when the target group is not assignable

[takano32]

In user-namespace environments (e.g. ChromeOS Crostini) getgroups(2) can
report supplementary groups such as the overflow GID 65534 (nobody) that a
non-root process cannot actually chgrp a file to.

TestFileUtils#setup builds @groups straight from
[Process.gid] | Process.groups, so the chown tests try to chgrp a file to
such a group and fail with EPERM instead of being skipped:

File.chown(nil, @groups[1], path)  # => Errno::EPERM

This filters @groups with a one-time capability probe down to the groups the
process can actually assign, so the affected tests skip via their existing
return unless @groups[1] guard when there is no usable second group. On
systems where the group is usable (or as root) the tests run exactly as before.

[takano32]

Gentle ping on this one 🙏

Just to add some concrete context on the motivation: this isn't a hypothetical edge case — make test for fileutils currently fails out of the box on ChromeOS Crostini (and other user-namespace container setups), because getgroups(2) reports the overflow GID (65534/nobody) as a supplementary group that the process can't actually chgrp to. The chown tests then raise Errno::EPERM instead of skipping.

This PR is test-only, has no impact on the library itself, and just filters @groups down to groups that are actually assignable so the existing return unless @groups[1] guard can skip cleanly. On systems where the group is usable (or as root) the tests run exactly as before.

Happy to adjust the approach if you'd prefer something lighter than the probe-based check. Thanks for taking a look!