ln: add error handling for hard link creation on directories

[FidelSch]

Adresses #9333

[sylvestre]

Lgtm (waiting for the ci now)

[cakebaker]

A detail: you can simplify the code a bit by using stderr_contains:

Suggested change

	let result = scene.ucmd().args(&["dir", "dir_link"]).fails();
	assert!(
	result
	.stderr_str()
	.contains("hard link not allowed for directory")
	);
	scene
	.ucmd()
	.args(&["dir", "dir_link"])
	.fails()
	.stderr_contains("hard link not allowed for directory");

[FidelSch]

totally missed that, adding it now

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/ln/hard-to-sym. tests/ln/hard-to-sym is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/ln/hard-to-sym. tests/ln/hard-to-sym is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/ln/hard-to-sym. tests/ln/hard-to-sym is passing on 'main'. Maybe you have to rebase?

[FidelSch]

GNU tests seem to not pass still, even though I am mimicking the behaviour of my builtin ln.
Tests seem to expect a different error message. Maybe from a different coreutils version?

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/ln/hard-to-sym. tests/ln/hard-to-sym is passing on 'main'. Maybe you have to rebase?

[cakebaker]

@FidelSch we replace the error message used in the GNU test in util/build-gnu.sh

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/fold/fold is no longer failing!

[cakebaker]

Thanks for your PR!