mv: gnu test case to-symlink fix

[matrixhead]

tries to fix #6577

Behaviors changed

• mv would remove the destination file first, if we are trying to move the file between different partitions and the destination is a symlink

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/timeout/timeout. tests/timeout/timeout is passing on 'main'. Maybe you have to rebase?
Congrats! The gnu test tests/mv/to-symlink is no longer failing!

[BenWiederhake]

Code looks fine, and the test would detect most types of bugs. Great!

The remaining CI failures are unrelated issues: GNU test timeout is flaky, #6534, #6570 (both).

Can I ask you to improve the error message though? Right now it's very cryptic. Also, the test could use an additional check, just to make extra sure that we will never accidentally overwrite other_fs_file.txt or something.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/mv/to-symlink is no longer failing!

[BenWiederhake]

Looks good, the only remaining CI failure is unrelated: #6534.

I'm a bit unhappy about the many fully-qualified usages of various traits/structs, like std::io::Write::write_all and std::os::unix::fs::symlink etc. However, I couldn't find a different test that has to deal with absolute filenames like this, and I can't come up with a better way to do it, so I just accept it and merge it.

If you can come up with a nicer way in the future, please do create a PR :)

[matrixhead]

okay, I will try 😃