Command tests (#922)

* WIP command tests

* Finish marking todo tests

* update

* update

* Windows cd test ignoring

crates/nu-command/tests/commands/math/avg.rs

@@ -0,0 +1,27 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn can_average_numbers() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+             open sgml_description.json
+             | get glossary.GlossDiv.GlossList.GlossEntry.Sections
+             | math avg
+         "#
+    ));
+
+    assert_eq!(actual.out, "101.5")
+}
+
+// FIXME: jt: needs more work
+#[ignore]
+#[test]
+fn can_average_bytes() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats",
+        "ls | sort-by name | skip 1 | first 2 | get size | math avg | format \"{$it}\" "
+    );
+
+    assert_eq!(actual.out, "1.6 KB");
+}