Adapt and add slowness test from @JohnMcLear.

- The test now ensures that all three key events are fired when
  sending keys. Previously, only the 'keypress' event was sent,
  which failed to trigger very slow code on webkit browsers (as
  it is triggered by 'keyup'). All three events should really be
  sent whenever sending keys to the browser to ensure that we're
  adequately testing real behavior. See the 'sendkeys' plugin
  for more; it only sends 'keypress'.