Command tests (#922)

* WIP command tests

* Finish marking todo tests

* update

* update

* Windows cd test ignoring

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

@@ -0,0 +1,15 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn adds_a_row_to_the_end() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+                echo  [ "Andrés N. Robalino", "Jonathan Turner", "Yehuda Katz" ] 
+                | append "pollo loco"
+                | nth 3
+        "#
+    ));
+
+    assert_eq!(actual.out, "pollo loco");
+}