[Jenkins] Listener port "zero" fixed (probably)

Listener was launched with port 0, which caused impossibility to connect. The code seemed OK, so I just did some small refactoring and it works now (which makes me :confused:)


TEST
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Run power tests with listener enabled and look in `ps aux` or `htop` whether the port specified for script is not zero. Run the tests on fresh machine so it can be set properly. Tests should not error and some events should be caught in the HTML, obviously)