add an argument that sends a test notification

[mscurtescu]

Is your feature request related to a problem? Please describe.

It is difficult to test notification settings, you have to wait for an event to see if they work.

Triggering an event is non trivial.

Describe the solution you'd like

Add an argument similar to --run-once, for example --test-notifications that only sends out a test message.

Describe alternatives you've considered

I tried to directly call the Shoutrrr testing tool, but it is not clear if it is actually available in the image and at what location.

Additional context

No response

[github-actions]

Hi there! 👋🏼 As you're new to this repo, we'd like to suggest that you read our code of conduct as well as our contribution guidelines. Thanks a bunch for opening your first issue! 🙏