Feat: Improve Notif. Test message to catch edge cases

[chakflying]

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Description

In the past, there were a few issues where problems with peoples' notification configuration caused UP/DOWN notification to fail to deliver, but the manually triggered Test message is unable to reproduce the issue. Just from memory key ones are:

• Handling newline characters within msg: e.g. a DOWN message for a Ping monitor would most likely include a newline character. This has caused webhook notifications to fail to deliver because of issue with templating.
• Unicode char: The cross emoji somehow cause the message to be blocked by notification providers.

Maybe we should improve the test message to include these common pitfalls for better testing.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas (including JSDoc for methods)
• My changes generates no new warnings
• My code needed automated testing. I have added them (this is optional task)

Screenshots (if any)

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[CommanderStorm]

Another change that might be worth lumping into this is to make the Test-button iclude a monitorJSON, as all other notifications (Cert-Exp, UP, DOWN) have that.

See #3414