Fetch a sample data before testing locate() function

[santisoler]

After Pooch v1.1.1 the cache directory where all sample data are downloaded to
is created when a dataset is fetched and not on initialization as before. This
makes the test_datasets_locate() to fail on a fresh installation of Harmonica
(where no dataset has been downloaded yet). The solution proposed in Verde
(fatiando/verde#271) is to fetch one dataset before
running the locate() function.

Fixes #192

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.