Include HTTP server/client in the mock Xaya servers

[domob1812]

We have ready-made classes for mocked RPC servers in unit tests (e.g. MockXayaServer), but using them still requires setting up the JSON-RPC HttpServer and HttpClient specifically in each test. Instead, we should bundle them together, perhaps with some kind of wrapper template class. This should help us remove quite a lot of boilerplate code in test fixtures.