tdml:parserTestCase that has neither tdml:infoset or tdml:error causes Abort

If you have a tdml test case that doesn't have either a tdml:infoset or tdml:error tag, then you get the following unhelpful invariant abort error:

Abort: Invariant broken. Should be Some None, or None Some only.

Seems like this is expecting this check to happen somewhere else, but it's not happening. We either need to ensure this check happens somewhere else, or change this abort into a more user friendly TDMLException with helpful error message.