Add unit tests to pallet-xcm (#3539)

* init

* reserve transfer asset unit test

* happy path tests for all dispatchables and can teleport assets to self surprisingly enough

* better tests

* minimize mock

* clean

* test send xcm failure when router blocks

* rm numbers to prefer variables

* clean

* test execute with local effects

* rm local env stuff

* copyrights

* fix merge conflict

* appease spellcheck

* fmt

* address review comments

* clean constants

* fmt

polkadot/xcm/pallet-xcm/src/lib.rs

@@ -18,7 +18,11 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
-use codec::{Decode, Encode};
+#[cfg(test)]
+mod mock;
+#[cfg(test)]
+mod tests;
+
 use frame_support::traits::{Contains, EnsureOrigin, Filter, Get, OriginTrait};
 use sp_runtime::{traits::BadOrigin, RuntimeDebug};
 use sp_std::{boxed::Box, convert::TryInto, marker::PhantomData, prelude::*, vec};