test: skip doctool tests when js-yaml is missing

Skip the doctool tests when js-yaml, which is currently `require()`d
from the eslint source tree, is missing. This can happen, for example,
because eslint is not included in the release source tarballs.

Fixes: https://github.com/nodejs/node/issues/7201
Ref: https://github.com/nodejs/node/pull/6495
PR-URL: https://github.com/nodejs/node/pull/7218


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>