Update test runner to match sass-spec

- The sass-spec runner by default looks for `input.json` and will skip
  any test which doesn't have one. Mirror that behaviour by skipping the
  test run if an `input.json` file doesn't exist.
- The sass-spec folders have been moved around so update the ignored
  folders to match where the todo tests now live.
Merge pull request #457 from edds/update-test-runner

Update test runner to match sass-spec