Save the last successful backup and add --last-backup

Write the completion time and backup directory to
$XDG_STATE_HOME/simple_backup/last_backup after each successful backup
(as the target user when run as root), for status bar reminders.
--last-backup prints it.

tests/conftest.py

@@ -3,6 +3,7 @@
 import sys
 from collections.abc import Callable, Iterator
 from datetime import datetime, timedelta
+from pathlib import Path
 
 import pytest
 
@@ -10,13 +11,14 @@ from simple_backup import simple_backup as sb
 
 
 @pytest.fixture(autouse=True)
-def no_side_effects(monkeypatch: pytest.MonkeyPatch) -> None:
-    """Keep tests out of the systemd journal and the desktop notifications"""
+def no_side_effects(monkeypatch: pytest.MonkeyPatch, tmp_path: Path) -> None:
+    """Keep tests out of the systemd journal, the desktop notifications and the user's state directory"""
 
     if hasattr(sb, 'j_handler'):
         sb.logger.removeHandler(sb.j_handler)
 
     monkeypatch.setattr(sb, '_notify', lambda text: None)
+    monkeypatch.setenv('XDG_STATE_HOME', str(tmp_path / 'state'))
 
 
 @pytest.fixture