Tweak: track files to test using `Vows.suites`

I used the “suite” term for consistency with the [PyVows website](http://pyvows.org).

Currently, `Vows.suites` is unused; however, I think it has potential to be quite helpful in the future.  

Since `Vows.suites` is created just once—at the same time that `Vows` is iterating over files —it should have nearly zero impact on performance.