api/tests: add pytest plumbing for testing rpc client/server stack as a whole

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.