additional test helpers

- assertNoFile: asserts that a file does not exist, test will fail
  if the file is present
- assertFileHasNoContent: asserts that a particular file does not
  have the content provided in the regex
- sharedExamples: allows re-use of common examples in test to help
  remove duplication and ease the burden in control flow branches when
  building the generator. Use sparingly. If used a lot, perhaps the
  generator is to complicated and should be re-thought at a higher level

/cc @eastridge, @ryan-roemer - let me know if these are okay to add. I am using them as i pull in testing stuff so I figured they we're useful on their own. I'm not sure if they should stay around for the long term but refactoring the generator while adding the testing stuff is much faster this way