Refactor shell listing related code (#6262)

* Refactor shell listing related code

Signed-off-by: nibon7 <nibon7@163.com>

* add test

Signed-off-by: nibon7 <nibon7@163.com>

crates/nu-command/tests/commands/shells.rs

@@ -0,0 +1,31 @@
+use nu_test_support::{nu, pipeline, playground::Playground};
+
+#[test]
+fn list_shells_1() {
+    Playground::setup("list_shells_1", |dirs, sandbox| {
+        sandbox.mkdir("foo").mkdir("bar");
+
+        let actual = nu!(
+            cwd: dirs.test(),
+            pipeline(
+            r#"enter foo; enter ../bar; g| get active.2"#
+        ));
+
+        assert_eq!(actual.out, "true");
+    })
+}
+
+#[test]
+fn list_shells_2() {
+    Playground::setup("list_shells_2", |dirs, sandbox| {
+        sandbox.mkdir("foo").mkdir("bar");
+
+        let actual = nu!(
+            cwd: dirs.test(),
+            pipeline(
+            r#"enter foo; enter ../bar; shells| get active.2"#
+        ));
+
+        assert_eq!(actual.out, "true");
+    })
+}