Summary of changes in UCommand:
* Extend UCommand by builder methods and simplify methods in TestScenario
* Simplify code structures where possible. Add documentation.
* Store bin_path as PathBuf and util_name as String in all structs
* Remove UCommand::util and make bin_path, temp_dir private
* Rename UCommand::with_limit -> UCommand::limit
Summary of changes in TestScenario:
* Rename some parameters in TestScenario methods to be more descriptive
* Remove ucmd_keepenv, cmd_keepenv from TestScenario. Use UCommand::keep_env instead.
`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 `'`
