Command tests (#922)

* WIP command tests

* Finish marking todo tests

* update

* update

* Windows cd test ignoring

crates/nu-command/tests/format_conversions/bson.rs

@@ -0,0 +1,17 @@
+#[cfg(feature = "bson")]
+#[test]
+fn table_to_bson_and_back_into_table() {
+    use nu_test_support::{nu, pipeline};
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            open sample.bson
+            | to bson
+            | from bson
+            | get root
+            | get 1.b
+        "#
+    ));
+
+    assert_eq!(actual.out, "whel");
+}