Refactor the `tip.parser-test` namespace

There was a lot of duplicated code, which wasn't very pretty and violated the DRY principle. The addition of a few helper functions has greatly cleaned up the namespace.

This relates to issue #4.
Correct copy-paste error

I accidentally turned a negative test into a positive one, making the test suite fail. This corrects this issue.

Related to issue #4.
Merge pull request #7 from emiln/fix/4-dry-tests

Refactor the `tip.parser-test` namespace