Command tests (#922)

* WIP command tests

* Finish marking todo tests

* update

* update

* Windows cd test ignoring

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

@@ -0,0 +1,37 @@
+#[test]
+fn selects_a_row() {
+    Playground::setup("nth_test_1", |dirs, sandbox| {
+        sandbox.with_files(vec![EmptyFile("notes.txt"), EmptyFile("arepas.txt")]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                ls
+                | sort-by name
+                | nth 0
+                | get name
+            "#
+        ));
+
+        assert_eq!(actual.out, "arepas.txt");
+    });
+}
+
+#[test]
+fn selects_many_rows() {
+    Playground::setup("nth_test_2", |dirs, sandbox| {
+        sandbox.with_files(vec![EmptyFile("notes.txt"), EmptyFile("arepas.txt")]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                ls
+                | get name
+                | nth 1 0
+                | length
+            "#
+        ));
+
+        assert_eq!(actual.out, "2");
+    });
+}