DON'T MERGE: AnswerTests.Rd master help file

We've had several requests from swirl instructors to clarify options for answer testing. @WilCrofter has already done much of the work by creating an individual help file for each of the standard answer tests. I'd like to leverage this by creating a master help file that links to the individual help files.

For ease of implementation, I've created this master help file in the swirl package, which is where the current help files reside. Long term, I think it makes sense to create a separate package that contains all of the answer tests code and docs. This separate answer tests package would then become a dependency of both swirl and swirlify. I'd like to hear what everyone thinks about this.

You can pull up this help file with `?AnswerTests`. There's not much in there yet.