Summary of changes in tests/util:
* Introduce global constant TESTS_BINARY holding the path to `coreutils`
* Implement running an arbitrary command in a sh or cmd shell per default.
* Implement std::fmt::Display for UCommand.
* Change usages of UCommand::util_name from &Option<OsStr> to Option<OsStr>
* `UCommand::new_from_tmp`: Use OsStr directly instead of TempDir -> String -> OsStr
* Collect arguments in `UCommand::args` field
* Build environment variables in `UCommand` itself instead of `std::process::Command`
* Move building of std::process:Command from fields in UCommand to own method `build`
* Remove assertions of UCommand::has_run in arg, args and env.
Summary of changes in tests/by-util:
* Remove usages of UCommand::raw. Fix tests to use UCommand::to_string.
* test_test: Adjust test_invalid_utf8_integer_compare to use `UCommand::args`
* test_chmod: run_single_test
* test_pwd: symlinked_env
* test_cp:
Fix the usage of &Option<OsStr> in `test_src_base_dot`
Refactor test_src_base_dot to not use UCommand::new but ts.ucmd() instead
