Implement custom x/mock for customized test environment setup

[yoongbok-lee]

Summary

Currently we initialize all testing environment from scratch, like a new App for example. We should refer to x/mock in the CosmosSDK and unify the initialization process from there.

[hacheigriega]

I realized that it gets difficult to avoid import cycles when we use x/mock to test modules that require access to many other modules, like x/shield. Incidentally, I think this is one of the reasons why Cosmos has transitioned to simapp. cosmos/cosmos-sdk#4875

So perhaps we should use simapp as well if we want to do testing of integration of multiple modules?

[yoongbok-lee]

Hmm that makes sense. Let's use simapp. we can transition your branch into a PR.