[alexluong]

Run the deliverymq retry tests on a fake clock. Based on #1162.

deliverymq was the slowest package in the short suite: its tests ran one after another, each waiting out rsmq visibility timeouts and context deadlines. Everything now runs inside a synctest bubble (in-memory deliverymq, the retry scheduler on testutil.NewSynctestRedis, Monitor, mempubsub), with one shared teardown, stopRetryStack. The tests also run in parallel (t.Parallel); none of them touch package-level state.

The package now runs in under 1 s.

The conversion surfaced three test bugs:

1. TestRetryScheduler_EventFetchSuccess was flaky: it could assert after the 2nd publish but before the 2nd log entry was written (also a data race on the entries).
2. TestDeliveryMQRetry_RetryMaxCount published a task without Attempt (0); real tasks start at 1, so it made 4 attempts. The old check asserted as soon as it saw 3 and never waited for the 4th. Now Attempt: 1; setting it back to 0 fails with 4 vs 3.
3. mockEventGetter.records was written and read without its mutex.

Timing choices:

• WithRetryNotReadyDelay(time.Second): the 10 s default outlasted the test windows. It only shortens the wait; the tested behavior is the same.
• RetryMaxCount and EligibleForRetryTrue use a 2 s backoff instead of 50 ms. Under ~1 s a retry is due at once and can run before the current attempt is logged. The lookup accepts attempts up to 1 s older than the retry's attempt time (retryLookupSkew), so it finds the previous attempt and publishes the current attempt number a second time. That's a race in the retry path, left for a follow-up; 2 s keeps these tests off it and costs nothing on the fake clock.
• "No further attempts" checks advance the fake clock through the full window before asserting. EventNotFound and EventFetchError also assert the lookup ran.
• The mempubsub consumer receives on context.Background() and stops through Shutdown: stopping it through its context leaks gocloud's receive goroutine, which fails the bubble.

The package was stress-tested with repeated runs, plain and with -race.

Not in this PR: the sub-second backoff race above.

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