test(graph): wait for event metrics before asserting

The mock backend finished before the event counter was incremented,
allowing the assertion to run too early. Wait for the counter and stop
the consumer during cleanup.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f