Migrate test framework from zcash_client_sqlite to zcash_client_backend

[str4d]

zcash_client_sqlite::testing contains a TestBuilder that can be used to produce a TestState, on which a test can operate. The test framework is generic over a block cache, that can optionally be provided with a TestCache implementation for tests that require block scanning. However, it currently hard-codes zcash_client_sqlite::WalletDb as the wallet that is tested.

In order to make the tests reusable with other wallet backends, we should rework TestBuilder etc to be generic over a TestWallet trait, that exposes the necessary operations that we currently depend on concrete WalletDb methods for. The new generic TestBuilder parameter should also be bound on WalletRead, WalletWrite, etc. as appropriate (i.e. only when the test framework requires a given bound).

Once that refactor is done, we can then move the test framework to zcash_client_backend behind the test-dependencies feature flag, and depend on it from zcash_client_sqlite. This will then free us up to subsequently genericise and migrate individual tests from zcash_client_sqlite to zcash_client_backend as we wish.

[oxarbitrage]

FYI: I made some progress on this yesterday after discussing with @nuttycom that this is a good time. I will keep working on it and submit a PR soon (today or in the next few days). For easier review my current plan is to push 2 PRs, one with the all the refactoring needed inside zcash_client_sqlite and after we are happy with it, make another one with the actual move.

[oxarbitrage]

This is being worked on by @nuttycom in #1530 so i am unassigning myself.