bullet proof smtp (fix flaky test)

[supersven]

This is almost the same as #2818, but should fix the test instability by using a different way to create random ports for the test SMTP server (postie).

The solution is inspired by our MockServer.

The approach can be sub-summed as:

• Use bindRandomPortTCP of streaming-commons. I.e. don't deal with all the traps and pitfalls of primitive Socket handling; let the library do it.
• Once the Socket is open, use it (instead of the port number). This should prevent race-conditions on the port.

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[battermann]

LGTM, only nitpicks, feel free to ignore :)

[supersven]

LGTM, only nitpicks, feel free to ignore :)

@battermann , all "nitpicks" were considered useful. Thanks a lot for them!