https://github.com/uutils/coreutils/pull/3084 (2a333ab391) had some
missing coverage and was merged before I had a chance to fix it.
This PR adds some coverage / improved error messages that were missing
from that previous PR.
`LANGUAGE=C` is not enough, `LC_ALL=C` is needed as the environment
variable that overrides all the other localization settings.
e.g.
```bash
$ LANGUAGE=C id foobar
id: ‘foobar’: no such user
$ LC_ALL=C id foobar
id: 'foobar': no such user
```
* replace `LANGUAGE` with `LC_ALL` as environment variable in the tests
* fix the the date string of affected uutils
* replace `‘` and `’` with `'`
