Prepare for porting the event class into Rust, where the constructor
strictly validates that all format-required fields (depth, hashes,
origin_server_ts, auth_events, prev_events, ...) are present. Most
tests build minimal dicts that omit these fields because they only
care about the fields the test exercises. Introduce make_test_event
and make_test_pdu_event, which layer format-version-aware defaults
on top of caller-supplied fields so individual tests don't need to
spell out every required key.
Erik Johnston