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