Implement backend test helpers

Backend test helpers are server side components (part of OONIB) that assist the OONI probe clients in running tests.

Every time a client has intention of running a certain OONI test they will create a report with OONIB (over a Tor Hidden Service). If the test requires a test helper the client must also send it's IP address to the backend. This is done to allow the backend to keep track of which client is associated with which test report id.

Using the IP address of the client a globally unique identifier for keeping track of test session could create problems if two users behind NAT end up using the same test. For this reason we could add a way to update the report with some additional parameters that would allow more accurate tracking of the testing session, like for example the src port of TCP connections.

I would suggest we first implement the basic test helpers that use the IP address of the client as a reference and then add the IP:src_port pair based tracker at a later stage.

Here we should keep track of all the required test helpers