Integration test setup for firehose websocket

[DavidBuchanan314]

Probably want a async with wrapper that connects to the firehose and parses and stores the messages it receives, so that they can be checked for expected results afterwards. (will probably want to have it wait for a specific number of messages (with timeout) to avoid race conditions)

We also want to test the subscribeRepos endpoint itself (various cursor values, futureCursor). There are currently race conditions around backfill/livetail cutover, it'd be great to have a test case that actually detects this and fails (so we can be relatively confident when it's fixed)