x/leverage: Increase testing of msg response return values

[toteki]

Currently MsgSupply, MsgWithdraw, MsgRepay, and MsgLiquidate have tests in keeper_test.go which check their return values against expected values.

These tests could be improved by making them case-based (and increasing the number of situations covered) as well as checking the consistency of the returned values (e.g. collateral received) against actual changes in balances, to catch any scenarios where the effects of a transaction do not match its reported returns.