Consolidate tests and add GitHub Actions

[eumiro]

As discussed in #214, this adds the GitHub Actions config. Since there is no GHA in the dev branch until now, the result of the CI run won't be shown immediately here. This is the first version running tests only. Coverage/pyflakes/pycodestyle will be added as next.

If everything works well and the GHA config is completed, .travis.yml can be removed in the near future.

This also moves the test.py file to the new tests directory as test_gpxpy.py and the test_files directory is renamed to tests/testdata to comply with the standards and where they can be discovered by pytest (which can be later used in the project for more readable tests and better messages).

Would you agree to move gpxpy to the src/gpxpy directory as well?