Generate ClientCallUnary test stubs.

[mpetrov]

This simplifies testing for async calls, e.g. a failing request could
be simulated with something like:

  override func get(_ request: Echo_EchoRequest, metadata customMetadata: Metadata, completion: @escaping (Echo_EchoResponse?, CallResult) -> Void) throws -> Echo_EchoGetCall {
    completion(nil, .error(statusCode: .notFound))
    return Echo_EchoGetCallTestStub()
  }

[MrMage]

👍 This is how I wanted to avoid having to create an unused Channel in
#378 (see my review of that PR). That PR needs a few more cleanups; afterwards stubbing async calls should be fully supported.

[MrMage]

Does this need to be generic at all?

[mpetrov]

Not strictly necessary at the moment, was primarily going for consistency with ClientCallClientStreamingTestStub. The generics would be necessary if we want similar request queueing or response recording, thought TBH I find that approach a bit to constrained for my use-cases. In unit tests I'd generally want to test a lot of error cases and in integration tests I'd generally want the stub to have business logic (as in, I really want an in-memory fake).

I don't have strong preferences regarding the generics, let me know if you'd prefer I remove them. Happy to update the PR either way.

[MrMage]

Thanks for the elaboration! For now, let's remove them; I'm open to re-adding them later on if necessary.

[mpetrov]

Done.