Add notification test

[robled]

I saw a screenshot of a version with a "test notification" button, but perhaps it was removed in a more recent version. This option is very helpful for initial configuration, especially for notification services that require authentication.

[jayjoshi64]

I second this request. I am getting hard time getting to work with notifications. till now, all I ready is that it's supported by shoutrrr, nothing else. it would be good if there's a test notification button... and also some sort of config/description on what sort of notifications are actually supported? I have no clue what to expect from it. some examples would be great.