Tests without internet

[jayvdb]

Would it be possible for the tests which require network to be marked with an identifier like 'network' so that they can be skipped easily.

Currently I am running the tests on an rpm build worker as

%pytest -v -k 'not (test_has_license or test_has_no_misc_files or test_has_executable or test_is_valid or (TestIntegrationHelloWorld and test_run))'

c.f. WIP https://build.opensuse.org/package/view_file/home:jayvdb:py-new/python-cnab/python-cnab.spec

It would also be nice if the PyPI sdist included the test fixtures, but given the relative simplicity of them it might be better for them to be created dynamically by conftest.py