cli/engine: expose commands to facilitate testing the API

[bassosimone]

We discussed with @FedericoCeratto that it would be useful to have commands for testing the APIs in the same way in which the probe actually uses them. This basically means having a simple way of calling the right session method. It could either be a subcommand of ooniprobe or another private binary build from ./internal/cmd. Maybe the former is better.

BTW, we should ensure that we test the same codepath of a real probe.

[hellais]

This will probably happen as part of engine split work: #2826