Remove the use of build tags for testing

Follow up from some discussion in https://github.com/cosmos/evm/pull/387.

Right now a lot of our testing uses logic that is built with `-tags=test`. This creates pain when testing isolated functions and makes it so that sometimes we are not directly testing the logic we have written - we are testing some other compiled "test version".

Using normal rules around testing like _test packages and functions, we should be able to move logic away from using build tags and just into the native testing capabilities of Go.