Update unit tests based on the commonmark spec in every (full) build

[nicojs]

We should add a direct dev-dependency to the commonmark spec. During every build we should regenerate the test cases from it. That way, it is far easier to not get out-sync with the spec.

Right now, we are a little bit out-sync with the latest tests. At first, we should target a specific version of the spec, as we do not want to update the tests in the PR for this issue.