[testing] add suite of functional tests to perform against recsystems;

particularly against their JSON-RPC interface

while these tests are not useful for testing the recommendations themselves
(see the explanation in test_recommendations--this is something implementers
of individual recsystems are responsible for testing--it does provide a good
smoke test as to whether the recsystem has implemented all the JSON-RPC
interfaces correctly

it is designed to in principle be run against any recsystem written in any
language

the test itself is written in Python and requires Python + pytest to run, but
it runs the recsystem itself in a subprocess and can be adapted to any
executable providing the recsystem under test