Add conftest.py for the wall clock timer tests

For now, we include one fixture that we will use to mock the `datetime`
class imported in the wall clock timer definition, so we can control
the current time in the tests (and set the default to the UNIX epoch to
make it easier to read times in the tests).

We don't use time-machine for 2 reasons:

1. We will also need to use async-solipsism and time-machine also mocks
   the monotonic timer, sometimes conflicting with async-solipsism.
2. We want to tell only when sleep was called within the wall clock
   timer module, not in the tests for example.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

tests/timeseries/_resampling/wall_clock_timer/conftest.py

@@ -0,0 +1,21 @@
+# License: MIT
+# Copyright © 2025 Frequenz Energy-as-a-Service GmbH
+
+"""Fixtures for wall clock timer tests."""
+
+from collections.abc import Iterator
+from datetime import datetime
+from unittest.mock import MagicMock, patch
+
+import pytest
+from frequenz.core.datetime import UNIX_EPOCH
+
+
+@pytest.fixture
+def datetime_mock() -> Iterator[MagicMock]:
+    """Mock the datetime class in the target module and set now to the UNIX epoch."""
+    dt_symbol = "frequenz.sdk.timeseries._resampling._wall_clock_timer.datetime"
+    dt_mock = MagicMock(name="datetime_mock", wraps=datetime, spec_set=datetime)
+    dt_mock.now.return_value = UNIX_EPOCH
+    with patch(dt_symbol, new=dt_mock):
+        yield dt_mock