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 is much faster this way