Support for yield cancel

[brendt-napp]

Hello, I'd really like it if we could test something like the following

expectSaga(mySaga)
  .provide([
    [fork(otherSaga), fakeTaskObj]
  ])

  .fork(otherSaga)

  .cancel(fakeTaskObj)

my current workaround is to just use some helper functions and call those

export function cancelSelf() {
  return cancel();
}
export function cancelTasks(tasks = []) {
  return cancel(tasks);
}

and use them like so

expectSaga(mySaga)
  .provide([
    [fork(otherSaga), fakeTaskObj]
  ])

  .fork(otherSaga)

  .call(cancelTasks, fakeTaskObj)