Made cksum return an error if used on a directory.

[Ato2207]

This PR should fix #5805. Now an error is generated if cksum is used on a directory and it returns an exit code of 1.

[Ato2207]

Couldn't make the last test work by creating a temporary path + directory, so I used one already existing.

[cakebaker]

Sorry, a rebase is needed :|

For creating a temporary directory in test_all_algorithms_fail_on_folder you can use a TestScenario, something like:

let scene = TestScenario::new(util_name!());
let at = &scene.fixtures;
at.mkdir("some_name");

for algo in ALGOS {
    scene
        .ucmd()
        ...
}

[Ato2207]

Should be ok now I think, let me know if I need to do anything else.

[cakebaker]

Thanks!

It's almost complete, though there is still an issue with the example from #5805 if more than one item is specified. The show! macro might be helpful for this.

[Ato2207]

Wouldn't that be automatically solved when #5809 is merged?

[cakebaker]

No, #5809 is about showing an error when a specified file doesn't exist, whereas this PR is about showing an error when a directory is specified.

[Ato2207]

My bad, I understand.Should be ready now.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[cakebaker]

Thanks!