Fix port collision detection in functional tests

Summary: If a port is bound but not listening, cannot bind to it anymore but the test will see the port as closed and let us use that, causing a failure. This diff fixes this potential issue by trying to bind instead of connecting.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13433