A lot of tests depend on GNU's coreutils to be installed in order
to obtain reference values during testing.
In these cases testing is limited to `target_os = linux`.
This PR installs GNU's coreutils on "github actions" and adjusts the
tests for `who`, `stat` and `pinky` in order to be compatible with macOS.
* `brew install coreutils` (prefix is 'g', e.g. `gwho`, `gstat`, etc.
* switch paths for testing to something that's available on both OSs,
e.g. `/boot` -> `/bin`, etc.
* switch paths for testing to the macOS equivalent,
e.g. `/dev/pts/ptmx` -> `/dev/ptmx`, etc.
* exclude paths when no equivalent is available,
e.g. `/proc`, `/etc/fstab`, etc.
* refactor tests to make better use of the testing API
* fix a warning in utmpx.rs to print to stderr instead of stdout
* fix long_usage text in `who`
* fix minor output formatting in `stat`
* the `expected_result` function should be refactored
to reduce duplicate code
* more tests should be adjusted to not only run on `target_os = linux`
This is a refactor to reduce duplicate code, it affects chmod/ls/stat.
* merge `stat/src/fsext::pretty_access` into `uucore/src/lib/feature/fs::display_permissions_unix`
* move tests for `fs::display_permissions` from `test_stat::test_access` to `uucore/src/lib/features/fs::test_display_permissions`
* adjust `uu_chmod`, `uu_ls` and `uu_stat` to use `uucore::fs::display_permissions`
