Failing spec for listenToOnce mem. leak

This is an example of a failing test demonstrating #3226.

Obv. this wouldn't be merged, but it shows the problem.
No preference here. I chose to use the format from [L177](https://github.com/jashkenas/backbone/pull/3341#discussion-diff-18966735R177) for consistency. I can go through and refactor any tests with that if you'd prefer, but maybe that should be a separate commit?