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/Cargo.lock

@@ -5450,11 +5450,17 @@ dependencies = [
  "frame-support",
  "frame-system",
  "log",
+ "pallet-balances",
  "parity-scale-codec",
+ "polkadot-parachain",
+ "polkadot-runtime-parachains",
  "serde",
+ "sp-core",
+ "sp-io",
  "sp-runtime",
  "sp-std",
  "xcm",
+ "xcm-builder",
  "xcm-executor",
 ]