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

[rvagg]

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
d5863bc (breaking all CI)

/cc @nodejs/build @jasnell @thealphanerd

[rvagg]

https://ci.nodejs.org/job/node-test-commit/1516/

[MylesBorins]

works locally LGTM

[rvagg]

CI all passed, landed with some urgency as 7c9cdb0 so we can get CI working again, please don't let that stop further review! Changes can be added to #4412.

[Trott]

Since we've now had a situation where a doc-only commit broke CI, is it worth revisiting the "no need to run doc-only PRs through CI" practice? I'd be all for a "every PR goes through CI, no exceptions" policy. Sure, it's rare and surprising when something like that breaks tests, but that's exactly the type of thing CI is there to surface.

Alternatively, we could get rid of the "test the code in the docs" business in test/addons. I'm less enthusiastic about that as I think it's valuable to run the documentation examples through some checking to make sure there aren't errors. All for making the code that handles it more robust, though.

Or we can carry on as before as this wasn't that big a deal in the grand scheme of things. I'd prefer to seize an opportunity to reduce our tolerance for glitches though.

[jbergstroem]

Can we reduce the amount of stuff we'd build/test for in a doc only PR? When we introduced the merge-pr jenkins job we had options for doc-only PR's; can perhaps we can do something similar with node-test-pr?

[Trott]

@jbergstroem Ooh, yes, that would be great. Just run the doc-relevant stuff and maybe only on a subset of platforms.

[jbergstroem]

@Trott could you create an issue in nodejs/build about it? I can do it otherwise.

[Trott]

@jbergstroem Done. nodejs/build#287