message: Document file sorting
diff: A smaller version of compiletest-rs
  +## Magic behavior
  +
  +* Tests are run in order of their filenames (files first, then recursing into folders).
  + So if you have any slow tests, prepend them with a small integral number to make them get run first, taking advantage of parallelism as much as possible (instead of waiting for the slow tests at the end).
  +
  ## Supported magic comment annotations
  Note that the space after `//`, when it is present, is *not* optional -- it must be exactly one.
  