chgrp: make test_reference less strict on Linux

[cakebaker]

The test_reference test recently started to fail in the CI on Linux with:

thread 'test_chgrp::test_reference' (14446) panicked at tests/by-util/test_chgrp.rs:210:14:
    assertion failed: `(left == right)`

    Diff < left / right > :
     chgrp: changing group of '/etc': Operation not permitted
    <failed to change group of '/etc' from runner to root
    >failed to change group of '/etc' from root to root

This PR makes the test less strict by no longer testing for specific groups.