Sort and separate the tests scripts about gmt

[leouieda]

To run these things on CI, it would be good to be able to specify different levels of tests that we want to run. Tests that are run on every build should be fast, ideally taking no more than 10min to run in total. We can run the full test suite with the slower tests on a nightly cron job. So it would help if we could "make test-fast" and "make test-full", for example.

[leouieda]

cc @joa-quim @seisman

[seisman]

Inspired by https://github.com/obspy/obspy_github_api and https://github.com/obspy/obspy/blob/master/.travis.yml.

Maybe we can separate it into several tests, and use some magic comment strings like +DOCS, +TESTS:FULL, +TESTS:0 to trigger specific tests.

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed if no further activity occurs within 7 days. Thank you for your contributions.

[joa-quim]

Closing on a too old and stalled criteria.