Unit tests fail without systest environment variables

[tseaver]

Running in a "clean" environment:

___________________ test_conformance_post_policy[test_data0] ___________________

test_data = {'description': 'POST Policy Simple', 'policyInput': {'bucket': 'rsaposttest-1579902670-h3q7wvodjor6bc7y', 'expiration...3/auto/storage/goog4_request', ...}, 'url': 'https://storage.googleapis.com/rsaposttest-1579902670-h3q7wvodjor6bc7y/'}}

    @pytest.mark.parametrize("test_data", _POST_POLICY_TESTS)
    def test_conformance_post_policy(test_data):
        import datetime
        from google.cloud.storage.client import Client
    
        in_data = test_data["policyInput"]
        timestamp = datetime.datetime.strptime(in_data["timestamp"], "%Y-%m-%dT%H:%M:%SZ")
    
>       client = Client(credentials=_DUMMY_CREDENTIALS)

tests/unit/test_client.py:1847: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
google/cloud/storage/client.py:111: in __init__
    project=project, credentials=credentials, _http=_http
.nox/unit-3-6/lib/python3.6/site-packages/google/cloud/client.py:226: in __init__
    _ClientProjectMixin.__init__(self, project=project)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <google.cloud.storage.client.Client object at 0x7fe605cc77f0>
project = None

    def __init__(self, project=None):
        project = self._determine_default(project)
        if project is None:
            raise EnvironmentError(
>               "Project was not passed and could not be "
                "determined from the environment."
            )
E           OSError: Project was not passed and could not be determined from the environment.

.nox/unit-3-6/lib/python3.6/site-packages/google/cloud/client.py:181: OSError
------------------------------ Captured log call -------------------------------
WARNING  google.auth._default:_default.py:334 No project ID could be determined. Consider running `gcloud config set project` or setting the GOOGLE_CLOUD_PROJECT environment variable
...

and likewise for the other parameterized test_conformance_post_policy tests.