Write a better test using mocking to test the behavior when a change stream is closed by the user with close after calling next, but before the call to processNewChange initiated by next has called back.

• call next on the change stream immediately
• call close on the change stream immediately
• it takes 10s for next to complete, but changeStream.closed is set to true immediately
• 10s later, the callback for next is triggered

Test should not: return a valid result after 10s
Test should: immediately callback indicating the stream has been closed

Likely will need to stub Topology.prototype.selectServer and maybe Topology.prototype.command.