Tests Refactor v1: remove mock

[rhuairahrighairidh]

This updates the tests to the almost-latest sdk testing style (cosmos/cosmos-sdk#4875).
Main aim was to remove mock to help with the move to module accounts (mock doesn't have a supply keeper).
Leaving update to using TestSuites (cosmos/cosmos-sdk#5257) to a future refactor.

Main changes:

• Moved tests over to using TestApp (a wrapper around the main app) rather than setting up all the keepers they need in a test_common.go.
• Changed initialization of TestApps in tests to use genesis states, rather than purely calling keeper methods. Not entirely sold on this change, as tests get more complex it might be easier to go back to initialization through keeper methods.
• converted auction/app_test.go to a keeper test

TODOs for future refactors:

• consider removing app_test.go style tests, they test things covered by keeper, handler, and cli tests
• consider moving TestApp and helper funcs to a new package to split out of app/
• maybe update validator-vesting tests

Progress:

• cdp
• liquidator
• pricefeed
• auction

[karzak]

Note. The use of package_test and explicit importing of public methods on package in tests is good hygiene, but I don't think we need to strictly follow that pattern in cases where we just want simple unit tests (including unit tests of private functions).