Unit tests must not depend on environment variables

[tseaver]

E.g.,:

$ env | grep GOOGLE || echo NO
NO
$ nox -re unit-3.9 -- -x
_______________ Test_Blob.test_download_as_byte_w_custom_timeout _______________

self = <tests.unit.test_blob.Test_Blob testMethod=test_download_as_byte_w_custom_timeout>

    def test_download_as_byte_w_custom_timeout(self):
>       self._download_as_bytes_helper(raw_download=False, timeout=9.58)

tests/unit/test_blob.py:1714: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tests/unit/test_blob.py:1557: in _download_as_bytes_helper
    client = self._make_client()
tests/unit/test_blob.py:59: in _make_client
    return Client(*args, **kw)
google/cloud/storage/client.py:122: in __init__
    super(Client, self).__init__(
.nox/unit-3-9/lib/python3.9/site-packages/google/cloud/client.py:277: in __init__
    _ClientProjectMixin.__init__(self, project=project, credentials=credentials)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <google.cloud.storage.client.Client object at 0x7fb9f31415b0>
project = None, credentials = None

    def __init__(self, project=None, credentials=None):
        # This test duplicates the one from `google.auth.default`, but earlier,
        # for backward compatibility:  we want the environment variable to
        # override any project set on the credentials.  See:
        # https://github.com/googleapis/python-cloud-core/issues/27
        if project is None:
            project = os.getenv(
                environment_vars.PROJECT,
                os.getenv(environment_vars.LEGACY_PROJECT),
            )
    
        # Project set on explicit credentials overrides discovery from
        # SDK / GAE / GCE.
        if project is None and credentials is not None:
            project = getattr(credentials, "project_id", None)
    
        if project is 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-3-9/lib/python3.9/site-packages/google/cloud/client.py:228: OSError

[tseaver]

I'm working toward closing this issue in the PRs for the refactoring in #38:

• refactor: add / use 'Client._get_resource' method #431
• refactor: add / use 'Client._patch_resource' method #436
• refactor: add / use 'Client._put_resource' method #441
• refactor: add / use 'Client._delete_resource' method #442
• refactor: add / use 'Client._post_resource' method #443
• refactor: add / use 'Client._list_resource' method #446

Once those are merged, there will be many fewer tests to clean up: almost all of them deal with blob downloads:

• tests: avoid requiring real credentials in unit tests #459