Add integration test for concurrent state and PR event sync

Implements test_concurrent_state_and_pr_sync which verifies that when both
a state event and a PR event for the same repository enter purgatory, both
are correctly synced from a remote relay and released when git data arrives.

The test:
1. Creates a source relay with a repo containing two commits
2. Sends both state event (referencing main branch) and PR event to source
3. Pushes git data to source relay, releasing both events from purgatory
4. Starts syncing relay that connects to source
5. Verifies both events sync to the syncing relay and are served
6. Confirms refs are correct (main branch and refs/nostr/<event-id>)

This validates that the purgatory sync mechanism handles multiple concurrent
events for the same repository without race conditions or conflicts.

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