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_;
 

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

@@ -0,0 +1,64 @@
+use std::path::Path;
+
+use itertools::Itertools;
+use nu_test_support::{fs::Stub, nu, playground::Playground};
+
+#[test]
+fn self_path_const() {
+    Playground::setup("path_self_const", |dirs, sandbox| {
+        sandbox
+            .within("scripts")
+            .with_files(&[Stub::FileWithContentToBeTrimmed(
+                "foo.nu",
+                r#"
+                    export const paths = {
+                        self: (path self),
+                        dir: (path self .),
+                        sibling: (path self sibling),
+                        parent_dir: (path self ..),
+                        cousin: (path self ../cousin),
+                    }
+                "#,
+            )]);
+
+        let actual = nu!(cwd: dirs.test(), r#"use scripts/foo.nu; $foo.paths | values | str join (char nul)"#);
+        let (self_, dir, sibling, parent_dir, cousin) = actual
+            .out
+            .split("\0")
+            .collect_tuple()
+            .expect("should have 5 NUL separated paths");
+
+        let mut pathbuf = dirs.test().to_path_buf();
+
+        pathbuf.push("scripts");
+        assert_eq!(pathbuf, Path::new(dir));
+
+        pathbuf.push("foo.nu");
+        assert_eq!(pathbuf, Path::new(self_));
+
+        pathbuf.pop();
+        pathbuf.push("sibling");
+        assert_eq!(pathbuf, Path::new(sibling));
+
+        pathbuf.pop();
+        pathbuf.pop();
+        assert_eq!(pathbuf, Path::new(parent_dir));
+
+        pathbuf.push("cousin");
+        assert_eq!(pathbuf, Path::new(cousin));
+    })
+}
+
+#[test]
+fn self_path_runtime() {
+    let actual = nu!("path self");
+    assert!(!actual.status.success());
+    assert!(actual.err.contains("can only run during parse-time"));
+}
+
+#[test]
+fn self_path_repl() {
+    let actual = nu!("const foo = path self; $foo");
+    assert!(!actual.status.success());
+    assert!(actual.err.contains("nu::shell::file_not_found"));
+}