test: test each block in addon.md contains js & cc

Allows more freedom in adding additional headings to addon.markdown,
otherwise it'll try and convert each block under a heading to a test
case. We need to have at least a .js and a .cc in order to have
something to test.

Fixes regression caused by adding a new 3rd-level heading in
d5863bc0

PR-URL: https://github.com/nodejs/node/pull/4411


Reviewed-By: Myles Borins <myles.borins@gmail.com>