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