tests: add testcase for the mqtt monitor with testcontainers

[CommanderStorm]

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Description

This issue is meant as a discussion.

I have thought a bunch about how our monitors can be tested and have come to the conclusion that for most monitors, testcontainers seem like a good fit.
Testing this part would allow us to merged PRs like #4445 or #4393 without the risk or the level of manual verification required.

Things that we should discuss:

• should new monitors be tested like this
• does addition of these tests add additional value? How important/is this important? (=> "do we want this? if yes, which merge slot")

Type of change

Please delete any options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Other
• This change requires a documentation update

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)

<- screenshot of testcases passing ->

[louislam]

Interesting. It is able to run on GitHub Actions?

[Zaid-maker]

Interesting. It is able to run on GitHub Actions?

Yes I guess so I tried it once

[CommanderStorm]

I don't see a reason why it cannot.
There might be some (minor) CI adaptations, but I can make + test those.