Feature Request: Support broadcasting messages as test users from the test host

[chatton]

From the tests, I would like to be able to broadcast arbitrary messages as users created in the tests themselves.

In issue #172 I found myself needing the private key of the users in order to sign transactions, however since then I found out that it should be possible to broadcast messages using this function.

If we use this function, I would no longer need private keys exposed (although I suspect it would still be useful in certain situations)

When attempting to utilize this function, I ran into a few issues.

• BroadCastTx signs the transactions by reading from the specified keyring, which means the test host needs access to valid keyring for this to work.
• A tx.Factory needs to be configured based on the chains.
• A client.Context needs to be configured based on user/chain/test environment.