test: Add tests for checkStorageOffline and checkGuestPoweredOff
- checkStorageOffline: 5 test cases covering confirmation polling,
alert creation, LastSeen updates, disabled storage handling
- checkGuestPoweredOff: 9 test cases covering confirmation polling,
alert creation, severity overrides, disabled guests, monitorOnly flag
Coverage: checkStorageOffline 0%→100%, checkGuestPoweredOff 0%→100%
Package coverage: 63.4%→65.4%
Note: Pre-existing test failure in suite related to /etc/pulse file
access - not caused by these changes.