Write tests for collect_chain_hashes

[teor2345]

In PR #1347, we implemented the FindBlocks and FindHeaders inbound peer requests.

Most of the code is quite simple. The only complex function is collect_chain_hashes.

We should test block_locator and collect_chain_hashes against each other using proptests.

Each proptest should make sure that collect_chain_hashes obeys the protocol rules.
(See the assertions in collect_chain_hashes for some of these rules.)

We might also want to add unit tests for protocol edge-cases.

I don't know if we'll have time to do these tests in the first alpha release.

[teor2345]

We have good test coverage for this code.