test: Add CheckGuest tests (41.4%→97.4%)

Cover all CheckGuest branches:
- Early return when alerts disabled
- Early return when all guests disabled
- VM and Container type handling
- Unsupported guest type returns early
- pulse-no-alerts tag suppresses alerts
- Stopped guest triggers powered-off check
- DisableAllGuestsOffline clears tracking
- Paused guest clears powered-off alert
- Non-running guest clears metric alerts
- Running guest clears powered-off alert
- Disabled thresholds clear existing alerts
- CPU, memory, disk metric checks
- Individual disk checks (mountpoint, device, index fallback)
- Skips disk with zero total or negative usage
- I/O metrics (diskRead, diskWrite, networkIn, networkOut)
- pulse-relaxed tag applies relaxed thresholds

Alerts package coverage: 76.0%→78.6%

Author: rcourtman