This PR adds a utility to wait for an address's balance to change. The utility checks for the balance change for two minutes, and returns the new value once it changes. It rreturns an error on timeout.

Some tests - particularly the ext-geth tests - are failing because balance changes that we expect to occur are not. My hunch is that this may be related to a race condition somewhere between the test code and op-geth, but I can't be sure. By waiting for the balance to change, we can eliminate spurious race conditions and see if this is related to some other underlying issue.