Add code to test test_that calls.

I wanted to make `test-xxx.r` actually testable, which was complicated by the fact that `test_that` is designed not to return anything. The natural fit seemed to be to pass in a reporter that did my bidding, which is what I've done. I think the result is that `test_test_that` works fairly reasonably, but if you think it's gotten too fancy for its own good, that's okay, too.

(Note that this branch is based on the other testing branch, so the code will look repetitive -- only the last diff is new. Is there a way to tell github to only show the relative changes? I'll clean this up once the other pull request is in.)