Implement end to end tests that attempt to propose invalid blocks

[evan-forbes]

With the introduction of ABCI++ #231, we are adding checks to proposal blocks. while we have tests to ensure that invalid proposals get rejected, we don't have an end to end test for this specific case.

[evan-forbes]

one strategy to accomplish this in both an integration or e2e test is to simply wrap the application and overwrite the prepare proposal method

[evan-forbes]

can close as completed with the addition of the malicious package, which adds the ability to propose and sign invalid blocks.

ensuring that ProcessProposal is always called is a different issue, and the rest of the test cases are covered by existing correctness testing