use MockXcm to simulate test

[zqhxuyuan]

currenty xcm-simulator and xcm-emulator both use non-relayed(thread-local/memory like) to test xcm.

As cumulus aleady support inject dmp_messages and hrmp_messages into MockXcm(also aim to test) in this PR: paritytech/cumulus#876. maybe we could let xcm-simulator unchanged
but xcm-emulator intergrate this feature so that we could also test something more than just handle_messages. because current the implementation of dmp-queue and xcmp-queue has some implements detail(such as cache,status,page etc), although the testcase in the cumulus repo can make sure it all works well. but I think we may need further/deeper integration?

[shaunxw]

Yes it would be helpful for tests. PR is welcomed.