System tests fail under emulator in clean environment

[tseaver]

Similar to googleapis/python-bigtable#431.

When the environment variable for the Firestore emulator is set, the client should disable any fallback lookups of credentials or project ID: they aren't needed for running under the emulator. At the moment, however:

$ env | grep CRED || echo "No credentials in environment"
No credentials in environment
$ env | grep PROJ || echo "No project in environment"
No project in environment
# Emulator running in separate shell -- no default creds available
$ which gcloud || echo "No gcloud"
No gcloud
$ export FIRESTORE_EMULATOR_HOST=localhost:8072
$ nox -s system -- -x
nox > Running session system-3.7
nox > Creating virtual environment (virtualenv) using python3.7 in .nox/system-3-7
nox > python -m pip install --pre grpcio
nox > python -m pip install mock pytest google-cloud-testutils pytest-asyncio -c /home/tseaver/projects/agendaless/Google/src/python-firestore/testing/constraints-3.7.txt
nox > python -m pip install -e . -c /home/tseaver/projects/agendaless/Google/src/python-firestore/testing/constraints-3.7.txt
nox > py.test --verbose --junitxml=system_3.7_sponge_log.xml tests/system -x
============================= test session starts ==============================
platform linux -- Python 3.7.12, pytest-6.2.5, py-1.10.0, pluggy-1.0.0 -- /home/tseaver/projects/agendaless/Google/src/python-firestore/.nox/system-3-7/bin/python
cachedir: .pytest_cache
rootdir: /home/tseaver/projects/agendaless/Google/src/python-firestore
plugins: asyncio-0.15.1
collected 94 items                                                             

tests/system/test_system.py::test_collections ERROR                      [  1%]

==================================== ERRORS ====================================
______________________ ERROR at setup of test_collections ______________________

    @pytest.fixture(scope="module")
    def client():
        credentials, project = _get_credentials_and_project()
>       yield firestore.Client(project=project, credentials=credentials)

tests/system/test_system.py:60: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
google/cloud/firestore_v1/client.py:99: in __init__
    client_options=client_options,
google/cloud/firestore_v1/base_client.py:129: in __init__
    _http=None,
.nox/system-3-7/lib/python3.7/site-packages/google/cloud/client.py:316: in __init__
    _ClientProjectMixin.__init__(self, project=project, credentials=credentials)
.nox/system-3-7/lib/python3.7/site-packages/google/cloud/client.py:264: in __init__
    project = self._determine_default(project)
.nox/system-3-7/lib/python3.7/site-packages/google/cloud/client.py:283: in _determine_default
    return _determine_default_project(project)
.nox/system-3-7/lib/python3.7/site-packages/google/cloud/_helpers.py:152: in _determine_default_project
    _, project = google.auth.default()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

scopes = None, request = None, quota_project_id = None, default_scopes = None

    def default(scopes=None, request=None, quota_project_id=None, default_scopes=None):
        """Gets the default credentials for the current environment.
        ...
>       raise exceptions.DefaultCredentialsError(_HELP_MESSAGE)
E       google.auth.exceptions.DefaultCredentialsError: Could not automatically determine credentials. Please set GOOGLE_APPLICATION_CREDENTIALS or explicitly create credentials and re-run the application. For more information, please see https://cloud.google.com/docs/authentication/getting-started

.nox/system-3-7/lib/python3.7/site-packages/google/auth/_default.py:488: DefaultCredentialsError
...
=============================== 1 error in 6.43s ===============================
nox > Command py.test --verbose --junitxml=system_3.7_sponge_log.xml tests/system -x failed with exit code 1
nox > Session system-3.7 failed.