[tests] refactor tests to use a markdown test fixture over RMarkdown to improve timing.

[zkamvar]

Before https://github.com/carpentries/sandpaper/releases/tag/0.6.0, if you wanted a markdown-only lesson, you had to do the following:

1. run no_package_cache()
2. run create_lesson()
3. edit episodes/01-introduction.Rmd to remove the R code chunk and rename it to episodes/01-introduction.md

Now, it's possible to avoid all of that by using rmd = FALSE in create_lesson(), which means that our testing of features unrelated to RMarkdown can be improved and sped up by starting with a markdown lesson (which copies the files instead of rendering them).