Simplify google calendar authentication setup (#67314)

Simplify google calendar authentication to combine some of the cases together, and reduce unecessarily checks. Make the
tests share common authentication setup and reduce use of mocks by introducing a fake for holding on to credentials.
This makes future refactoring simpler, so we don't have to care as much about the interactions with the credentials
storage.

tests/components/google/conftest.py

@@ -1,11 +1,17 @@
 """Test configuration and mocks for the google integration."""
+from __future__ import annotations
+
 from collections.abc import Callable
+import datetime
 from typing import Any, Generator, TypeVar
 from unittest.mock import Mock, patch
 
+from oauth2client.client import Credentials, OAuth2Credentials
 import pytest
 
 from homeassistant.components.google import GoogleCalendarService
+from homeassistant.core import HomeAssistant
+from homeassistant.util.dt import utcnow
 
 ApiResult = Callable[[dict[str, Any]], None]
 T = TypeVar("T")
@@ -36,6 +42,62 @@ def test_calendar():
     return TEST_CALENDAR
 
 
+class FakeStorage:
+    """A fake storage object for persiting creds."""
+
+    def __init__(self) -> None:
+        """Initialize FakeStorage."""
+        self._creds: Credentials | None = None
+
+    def get(self) -> Credentials | None:
+        """Get credentials from storage."""
+        return self._creds
+
+    def put(self, creds: Credentials) -> None:
+        """Put credentials in storage."""
+        self._creds = creds
+
+
+@pytest.fixture
+async def token_scopes() -> list[str]:
+    """Fixture for scopes used during test."""
+    return ["https://www.googleapis.com/auth/calendar"]
+
+
+@pytest.fixture
+async def creds(token_scopes: list[str]) -> OAuth2Credentials:
+    """Fixture that defines creds used in the test."""
+    token_expiry = utcnow() + datetime.timedelta(days=7)
+    return OAuth2Credentials(
+        access_token="ACCESS_TOKEN",
+        client_id="client-id",
+        client_secret="client-secret",
+        refresh_token="REFRESH_TOKEN",
+        token_expiry=token_expiry,
+        token_uri="http://example.com",
+        user_agent="n/a",
+        scopes=token_scopes,
+    )
+
+
+@pytest.fixture(autouse=True)
+async def storage() -> YieldFixture[FakeStorage]:
+    """Fixture to populate an existing token file for read on startup."""
+    storage = FakeStorage()
+    with patch("homeassistant.components.google.Storage", return_value=storage):
+        yield storage
+
+
+@pytest.fixture
+async def mock_token_read(
+    hass: HomeAssistant,
+    creds: OAuth2Credentials,
+    storage: FakeStorage,
+) -> None:
+    """Fixture to populate an existing token file for read on startup."""
+    storage.put(creds)
+
+
 @pytest.fixture
 def mock_next_event():
     """Mock the google calendar data."""