DON'T MERGE: experimenting with test lesson feature

Running into some bizarre issues here. Function `swirlify()` runs `swirl()` in `test` mode when the "Test Me!" button is pressed, which requires the `testlesson` branch of swirl to be built. For some reason, the "Test Me!" button must be pressed twice for swirl to begin. RStudio seems to crash often while I'm doing this, however the lesson does begin, which provides a glimpse of hope. 

Note that you must load swirl with `library(swirl)` before running this, since `install_course_directory()` calls `path.package("swirl")`, which apparently requires swirl to be fully loaded (not just by namespace).