Tunnel: Re-add unit tests for port mapping

[orgads]

This reverts commit b79e884 and implements it with proper mocking.

[orgads]

@connor4312 @alexr00 I rewrote the tests, hopefully with good mocking this time.

[alexr00]

@orgads, a big thank you for taking the time to re-add the tests! I'll wait to approve until I get your 👍 in case you want to use suiteSetup.

[orgads]

Thanks! That was really helpful.

Done, and the tests pass locally.