Demo unit test querier

[ethanfrey]

This starts showing how to unit test code with a fake querier and not always the full-stack multitest app

[hashedone]

Went through this, I am happy with this possibility, also think that it is good to have this test in repo so will merge.

One more think I would actually wish exist is implementation of Querier which is configurable like mocks/stubs (see - mocks in mockall crate). Basically so instead of creating my fake querier, there would be something like QuerierStub used like:

let querier = QuerierStub::new()
  .on(|query| /* some predicate */, |query| /* answer to this query if matches predicate*/);

[ethanfrey]

I agree.

Look at CosmWasm/cosmwasm#1050 for an issue in that direction. Happy if you add more ideas there.

We also could make such a class in cosmwasm-plus first, try it out in tests, and when we are happy with the api, make a pr to add it to cosmwasm