[test_framework] Better debug output during port collision

Summary: I spent some time figuring out a port collision on my machine, and this could be solved simply with a better debug output.

Test Plan:

ninja check-functional

Create a port collision (you can stop incrementing in the unique_port function) and check the output

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17460