extractor: Set age limit on some adult-related extractors.

More age limit of videos for adult-related sites.

Note that, for redtube, I explicitly left the variable containing the age
limit, since the comment justifying the age limit is a good thing to have.

That being said, I included the age limit field on the test, to better
reflect what the information extractor does (even if it may not break the
automated tests).

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>