Tests timing should be fixed

[wch]

The tests in test-client.R work if you step through them, but not if they're run all at once.

It does things like this, where last is set by the onMessage handler.

  ws$send(charToRaw("hello"))
  expect_identical(last, charToRaw("hello"))

The problem is that the expect_identical is called before the response is received. It would be nice to be able to add a line like waitfor(1), which returns if there's a response within one second (or if it has already occurred) and times out after one second.