Fix incorrect assertions in CanCancelTasks test

The asserted ordering of events was not in fact guaranteed.
We asserted that two executions waited, started, waited,
started; but in fact both could wait, then one could
start, then the other could start. Changed around the
assertions to test only orderings that should be
guaranteed.
Merge pull request #206 from sbt/wip/havocp-can-cancel-tasks-race

Fix incorrect assertions in CanCancelTasks test