`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 `'`
Calling `cmd_keepenv("mv")` spawned the system `mv` instead of
the uutils `mv`. Also, `keepenv` isn't necessary because it doesn't
need to inherit environment variables.
We now actually check the stderr, because previously the result of
`ends_with` was not used, making the test pass even when it shouldn't.
I disabled the test on windows because `mkdir` does not support `-m` on
windows, making the test fail because there will be no permission error.
On FreeBSD there isn't a permission error either, and `mv` succeeds.
