test,tools: test yaml parsing of doctool

Add checks that make sure the doctool parses metadata correctly.

PR-URL: https://github.com/nodejs/node/pull/6495


Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>