Remove integration tests where possible to replace with unit

[evan-forbes]

          I feel like we defer to using integration tests too often when unit tests would suffice

Originally posted by @cmwaters in #2515 (comment)

Unit tests are lighter, more precise, and easier to debug. We should reevaluate our rampant usage of the testnode integration tests and try to rewrite some of the tests with unit where possible.

We likely want to refactor our TestApp tooling so that its easier to write unit tests instead of having to resort to using integration tests just to execution transactions, have access to a mempool, or create new blocks.