Add utility functions for testing the wall clock timer

This commit adds a utility module for testing the wall clock timer. The
utility functions include a method to convert various time-related
types (datetime, timedelta, float) to seconds, and a function to get the
current wall clock time from the mocked datetime (when the
`datetime_mock` fixture is used).

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

Author: llucax

tests/timeseries/_resampling/wall_clock_timer/util.py

@@ -0,0 +1,36 @@
+# License: MIT
+# Copyright © 2025 Frequenz Energy-as-a-Service GmbH
+
+"""Utility functions for testing the wall clock timer."""
+
+from datetime import datetime, timedelta, timezone
+from typing import assert_never
+
+
+def to_seconds(value: datetime | timedelta | float) -> float:
+    """Convert a datetime, timedelta, or float to seconds."""
+    match value:
+        case datetime():
+            return value.timestamp()
+        case timedelta():
+            return value.total_seconds()
+        case float():
+            return value
+        case unexpected:
+            assert_never(unexpected)
+
+
+# This is needed to work with the datetime_mock fixture in conftest.py
+def wall_now() -> datetime:
+    """Get the current wall clock time from the mocked datetime in the target module."""
+    # Disable isort formatting because it wants to put the ignore in the wrong line
+    # We now also have to ignore the maximum line length (E501)
+    # isort: off
+    # pylint: disable-next=import-outside-toplevel
+    from frequenz.sdk.timeseries._resampling._wall_clock_timer import (  # type: ignore[attr-defined] # noqa: E501
+        datetime as mock_datetime,
+    )
+
+    # isort: on
+
+    return mock_datetime.now(timezone.utc)