Add a test that PrepareProposal returns the correct transactions in the response to Tendermint when proposing a block

[james-chf]

(Relates to https://github.com/anoma/namada/pull/494 but is more general than that PR)

When calling PrepareProposal with some txs from the mempool, we should check that the returned proposed block contains:

• all ProtocolTxType::VoteExtension protocol transactions that were passed to PrepareProposal
• appropriate vote extension digest transactions (i.e. ProtocolTxType::EthereumEvents and ProtocolTxType::ValidatorSetUpdate) based on the ProtocolTxType::VoteExtensions passed in
• roughly half of any wrapper transactions that were passed to PrepareProposal
• the rest of the returned transactions should be decrypted transactions

This can be a test in Rust code.