Test: Fix flaky cancellation test in challenge 30

[t4e1]

It seems to be an issue in challenge 30's test.

At TestProcessItems_Cancellation case, I need to add arbitrary time delay logic(time.Sleep(50 * time.Millisecond)) in my source code to pass the exam.

The test code should probably be revised.

[asarkar]

I've looked at some of the submissions for challenge 30, and those sleep for magical time to make the tests pass, like so:

time.Sleep(100 * time.Millisecond)