Improve --test behavior when the service is already running #87

Running:

school-bell schema.json --test

while school-bell.service is active fails with GPIO busy and prints a full traceback because both processes try to claim the same GPIO pins. In contrast, --check works while the service is active.

Expected behavior

Please clarify and implement the intended workflow:

• Require users to stop school-bell.service before using --test; or
• Support test mode through the running service.

At minimum, detect unavailable GPIO pins and show a friendly, actionable message instead of a traceback, for example:

GPIO pins are already in use, possibly by school-bell.service. Stop the service before running --test.

Environment

• school-bell: 0.9.2.post7
• Error: lgpio.error: 'GPIO busy'