testing: system test should test for produced epochs

[test_light_client_prover_talking()](https://github.com/deltadevsde/prism/blob/main/crates/tests/src/lib.rs) is our only system test and it isn't testing anything. It exits once the validator produces 100 heights, which ofc has nothing to do with prism.

It should track that at least X epochs have been produced, and the light node has verified at least Y of them. But the API for this doesn't exist yet, we need to add it to the prover and light node.