test(path self): Add tests (#14607)

# Description
Add tests for `path self`.

I wasn't very familiar with the code base, especially the testing
utilities, when I first implemented `path self`. It's been on my mind to
add tests for it since then.

crates/nu-command/tests/commands/path/mod.rs

@@ -4,6 +4,7 @@ mod exists;
 mod expand;
 mod join;
 mod parse;
+mod self_;
 mod split;
 mod type_;