Command tests (#922)

* WIP command tests

* Finish marking todo tests

* update

* update

* Windows cd test ignoring

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

@@ -0,0 +1,29 @@
+use nu_test_support::fs::Stub::FileWithContentToBeTrimmed;
+use nu_test_support::playground::Playground;
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn adds_a_row_to_the_beginning() {
+    Playground::setup("prepend_test_1", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "los_tres_caballeros.txt",
+            r#"
+                Andrés N. Robalino
+                Jonathan Turner
+                Yehuda Katz
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                open los_tres_caballeros.txt
+                | lines
+                | prepend "pollo loco"
+                | nth 0
+                "#
+        ));
+
+        assert_eq!(actual.out, "pollo loco");
+    })
+}