refactor: extract signal_alert into its own module

Move signal_alert out of python/common.py into a dedicated
python/signal_alert.py module and update its importers
(validate_system.py, snapshot_manager.py) to the new path.

Relocate the signal_alert tests from tests/test_common.py into
tests/test_signal_alert.py, repatching python.signal_alert.logger and
python.signal_alert.Apprise to match the new module.

python/system_tests/validate_system.py

@@ -9,7 +9,8 @@ from socket import gethostname
 
 import typer
 
-from python.common import configure_logger, signal_alert
+from python.common import configure_logger
+from python.signal_alert import signal_alert
 from python.system_tests.components import systemd_tests, zpool_tests
 
 logger = logging.getLogger(__name__)