bpo-26067: Do not fail test_shutil / chown when gid/uid cannot be resolved (#19032)

* bpo-26067: Do not fail test_shutil.chown when gid/uid cannot be resolved

There is no guarantee that the users primary uid or gid can be resolved
in the unix group/account databases. Skip the last part of the chown
test if we cannot resolve the gid or uid to a name.

* 📜🤖 Added by blurb_it.

* Address review feedback

* address review feedback correctly

* fix typo

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>