Improve test suite

There are many ways that the test suite could be improved. For example:

• The same inputs could be run with a variety of configs, like:
  • 2- or 4-space indents
  • 60, 80, or 120 width columns
  • free, left, or multiple indent policy
• Testing some larger files or groups of files, like:
  • Brittany itself (#238)
  • test suites from other projects like hindent
  • all of Stackage
  • all of Hackage