Fix intermittent test: TestNotifier/MultipleSubscribersStress

[brandur]

Try to fix one of the most commonly failing intermittent tests we have
left, which is the stress test in the notifier [1].

--- FAIL: TestNotifier (0.00s)
    --- FAIL: TestNotifier/MultipleSubscribersStress (0.98s)
        notifier_test.go:409: Generated schema "notifier_2025_04_26t20_08_50_schema_04" with migrations [1 2 3 4 5 6] on line "main" in 376.579906ms [4 generated] [0 reused]
        notifier_test.go:434: Sending notification on "test_topic1": msg0
        notifier_test.go:434: Sending notification on "test_topic1": msg1
        notifier_test.go:434: Sending notification on "test_topic1": msg2
        notifier_test.go:434: Sending notification on "test_topic1": msg3
        notifier_test.go:434: Sending notification on "test_topic1": msg4
        notifier_test.go:434: Sending notification on "test_topic1": msg5
        notifier_test.go:434: Sending notification on "test_topic1": msg6
        notifier_test.go:472: Channel  0 contains   5 message(s)
        notifier_test.go:472: Channel  1 contains   7 message(s)
        notifier_test.go:472: Channel  2 contains   0 message(s)
        notifier_test.go:477:
                Error Trace:	/home/runner/work/river/river/internal/notifier/notifier_test.go:477
                Error:      	Should NOT be empty, but was 0xc000244770
                Test:       	TestNotifier/MultipleSubscribersStress
        riverdbtest.go:277: Checked in schema "notifier_2025_04_26t20_08_50_schema_04"; 1 idle schema(s) [4 generated] [2 reused]
FAIL
FAIL	github.com/riverqueue/river/internal/notifier	2.025s

Rebuild the test a little so that each listen goroutine waits for at
least one message to come through instead of only an arbitrary sleep.
The sleep ~always passes locally, but it seems to be that in CI it's
possible for the sending goroutine to be entirely parked around when a
listen/unlisten loop is happening.

[1] https://github.com/riverqueue/river/actions/runs/14684674639/job/41211780010

[brandur]

Thanks!