Make sum plugin as internal command. (#1501)

crates/nu-cli/tests/commands/sum.rs

@@ -0,0 +1,30 @@
+use nu_test_support::fs::Stub::FileWithContentToBeTrimmed;
+use nu_test_support::playground::Playground;
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn all() {
+    Playground::setup("sum_test_1", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "meals.csv",
+            r#"
+                description,calories
+                "1 large egg",90
+                "1 cup white rice",250
+                "1 tablespoon fish oil",108
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                open meals.csv
+                | get calories
+                | sum
+                | echo $it
+            "#
+        ));
+
+        assert_eq!(actual, "448");
+    })
+}