.travis.yml: introduce concept of "extended tests"

Since CI is engaged on per merge request basis, it can be wasteful to
run each request through all the tests, especially those resource
consuming. Idea is to mark most of tests as "extended" and provide a
way to opt-in by marking last commit with [extended tests] tag. It's
still not as optimal as one could wish, as decision to skip a test
still requires machine time, and it's taken in configured environment,
i.e. with updates and additional packages installed...

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2292)