Motivation

* `testHalfCloseOwnOutputWithPopulatedBuffer` asserts that writing to a
  half-closed channel produces `ChannelError.outputClosed`. However, after
  flushing the buffered write and the FIN, the remote peer can process both
  and fully close the connection before the subsequent write is processed.
  When this happens the channel is fully closed and the write fails with
  `.ioOnClosedChannel` instead. This race seems to be more likely on iOS
  simulators.

Modifications

* Accept either `.outputClosed` or `.ioOnClosedChannel` as valid errors
  from the post-half-close write.

Result

* The test no longer flakes on iOS simulator CI runs.