Summary of Changes

While writing docs for RpcClient I wrote a bunch of integration tests against the TestValidator in order to understand how it behaved, as I didn't see any such tests in tree.

I am not sure the value of these: some of them amount to smoke tests, not asserting much; and they run slowly (it takes my laptop 654 seconds to run all these). I have looked into increasing the block time in the TestValidator but it is not a trivial change.

I understand if these aren't wanted. I am curious though whether the CI says they improve coverage.

Because of the way dependencies are organized, these are located in the solana_core crate, where TestValidator lives, not in the solana_client crate, where RpcClient lives.