Integration flake: TestTxnIDEcho (?)

[DMRobertson]

https://github.com/matrix-org/sliding-sync/actions/runs/4542347248/jobs/8005651044

❌ TestTxnIDEcho (0s)
  panic: send on closed channel
  
  goroutine 224 [running]:
  github.com/matrix-org/sliding-sync/pubsub.(*PubSub).Notify(0xc000449780, {0xb72ace?, 0xb0?}, {0xc7d840, 0xc0003d7140})
  	/home/runner/work/sliding-sync/sliding-sync/pubsub/pubsub.go:80 +0xd1
  github.com/matrix-org/sliding-sync/sync2/handler2.(*Handler).Accumulate(0xc0003b0cb0, {0xc00003ad40, 0x40}, {0xc00048ab70, 0xc}, {0x0, 0x0}, {0xc000036cc0, 0x1, 0x4})
  	/home/runner/work/sliding-sync/sliding-sync/sync2/handler2/handler.go:229 +0x363
  github.com/matrix-org/sliding-sync/sync2.(*PollerMap).Accumulate.func1()
  	/home/runner/work/sliding-sync/sliding-sync/sync2/poller.go:194 +0x7c
  github.com/matrix-org/sliding-sync/sync2.(*PollerMap).execute(0xc000075fb8?)
  	/home/runner/work/sliding-sync/sliding-sync/sync2/poller.go:183 +0x5b
  created by github.com/matrix-org/sliding-sync/sync2.(*PollerMap).EnsurePolling
  	/home/runner/work/sliding-sync/sliding-sync/sync2/poller.go:143 +0x158