Disable local Google Cloud credentials in tests (#6851)

Prevent tested notebooks from connecting to cloud services with
local user credentials.  Tell gcloud to use empty configuration
directory instead of user configuration within pytest session.

This makes the test of docs/tutorials/google/start.ipynb notebook
use virtual hardware engine unconditionally.

dev_tools/conftest.py

@@ -11,13 +11,15 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
 import os
 import shutil
 import sys
 import tempfile
 import uuid
 from pathlib import Path
 from typing import Tuple
+from unittest import mock
 
 import pytest
 from filelock import FileLock
@@ -26,6 +28,14 @@
 from dev_tools.env_tools import create_virtual_env
 
 
+@pytest.fixture(scope="session", autouse=True)
+def disable_local_gcloud_credentials(tmp_path_factory):
+    # Ensure tests cannot authenticate to production servers with user credentials
+    empty_dir = tmp_path_factory.mktemp("empty_gcloud_config", numbered=False)
+    with mock.patch.dict(os.environ, {"CLOUDSDK_CONFIG": str(empty_dir)}):
+        yield
+
+
 @pytest.fixture(scope="session")
 def cloned_env(testrun_uid, worker_id):
     """Fixture to allow tests to run in a clean virtual env.