Improve finalization of battery status tests

Use the `MockMicrogrid` as a context manager and a custom async context
manager for the `BatteryStatusTracker` so all used components get
automatically finalized.

Also uses `time_machine` as a `pytest` fixture when possible to simplify
the tests and avoid having too indent too deep, as the previous approach
used a context manager too, but not an async one, so it can be combined
in the same `with` statement.

This commit can be reviewed much more easily hiding differences in
spaces (with `diff -w`, or appending `?w=1` to the GitHub URL).

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

Author: llucax