Deprecate the verde.test function

[leouieda]

It's not needed and hardly ever used. The same functionality can be achieved with pytest --pyargs verde (testing the installed version). Will be removed in v2.0.0.

Related to fatiando/community#44

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[leouieda]

@santisoler we should probably do this for all other packages. It's a bit of a useless function and we don't even use it ourselves.