This uses both

- the `client` test fixture (from pytest-flask) to allow to easily call
  RPCServerApp based application methods,
- a `requests` Adapter class to make the RPCClient based proxy route the
  HTTP requests directly to the RPCServerApp test client above, without
  the need for a running HTTP server.

This adapter overload the `send()` method to call the Flake test app open()
method (instead of doing a real network connection), and convert the
wertzeug based Response object into a `requests` one.

Also add a few tests for the RPC client/server communication provided in
swh.core.api via RPCServerApp and RPCClient classes.