Make download tests robust to HEPData outages by mocking of tar.gz files

[matthewfeickert]

@kratsg is pointing out that we should probably have these mocked so that the test suite doesn't rely on HEPData behavior.

Example of something similar he's done recently

def test_user_expires_reauthenticate(user_temp, requests_mock, mocker):
    user, _ = user_temp
    assert user.is_authenticated()
    assert user.is_expired() == False
    assert user.expires_in > 0
    user._id_token['exp'] = time.time() - 1
    assert user.is_authenticated()
    assert user.is_expired()
    assert user.expires_in == 0

    mock = mocker.patch.object(user, '_parse_id_token')
    requests_mock.post(
        requests.compat.urljoin(itkdb.settings.ITKDB_AUTH_URL, 'grantToken'),
        text=json.dumps(
            {
                'id_token': {
                    'exp': time.time() + 3600,
                    'name': _name,
                    'uuidentity': _identity,
                },
                'access_token': _access_token,
            }
        ),
    )
    user.authenticate()
    user._id_token = {'exp': time.time() + 3600, 'name': _name, 'uuidentity': _identity}
    assert user.is_authenticated()
    assert user.is_expired() == False

Originally posted by @matthewfeickert in #1485 (comment)