kvserver/rangefeed: deflake TestUnbufferedRegOnCatchUpSwitchOver/disconnect_after_runOutputLoop

This test was using the capacity of the catchUpBuf to decide how many
post-catch-up events need to be delivered. This is a problem because the
the output loop might have already finished the catchup scan, flushed
its empty catch up buffer, set the buffer to nil (0 cap), and exited
before we ever even observe the cap of the catchup buffer.

One note is that this test is a bit odd because depending on the speed
of the catchup up scan, the events might come to us via the catchup
buffer publish or directly.

Fixes #152960
Release note: None

Author: stevendanna

pkg/kv/kvserver/rangefeed/unbuffered_registration_test.go

@@ -359,19 +359,17 @@ func TestUnbufferedRegOnCatchUpSwitchOver(t *testing.T) {
 			defer wg.Done()
 			r.runOutputLoop(ctx, 0)
 		}()
-		capOfBuf := cap(r.mu.catchUpBuf)
-		r.publish(ctx, ev1, &SharedBudgetAllocation{refCount: 1})
-		r.publish(ctx, ev2, &SharedBudgetAllocation{refCount: 1})
-		r.publish(ctx, ev3, &SharedBudgetAllocation{refCount: 1})
-		r.publish(ctx, ev4, &SharedBudgetAllocation{refCount: 1})
-		r.publish(ctx, ev5, &SharedBudgetAllocation{refCount: 1})
+		postCatchUpEvents := []*kvpb.RangeFeedEvent{ev1, ev2, ev3, ev4, ev5}
+		for _, ev := range postCatchUpEvents {
+			r.publish(ctx, ev, &SharedBudgetAllocation{refCount: 1})
+		}
 		catchUpEvents := expEvents(false)
 		wg.Wait()
 
 		require.False(t, r.getOverflowed())
 		require.Nil(t, r.getBuf())
-		s.waitForEventCount(t, capOfBuf+len(catchUpEvents))
+		s.waitForEventCount(t, len(postCatchUpEvents)+len(catchUpEvents))
 		require.Equal(t,
-			[]*kvpb.RangeFeedEvent{ev1, ev2, ev3, ev4, ev5}, s.mu.events[len(catchUpEvents):])
+			postCatchUpEvents, s.mu.events[len(catchUpEvents):])
 	})
 }