test: Use unassigned p2p_port instead of hardcoded 60000 in p2p_i2p_ports.py #33670

The goal is to fix #30030.

The root cause it unclear. However, hard-coding the port to 60000 does not seem ideal anyway. This could break in an unlikely setting where so many functional tests are run, such that the port is occupied. Also, it could fail when TEST_RUNNER_PORT_MIN is set sufficiently high. (This is purely theoretical, as I don't think anyone would run a command like this, but on current master it fails, and on this pull it passes: TEST_RUNNER_PORT_MIN=60000 ./bld-cmake/test/functional/p2p_i2p_ports.py --portseed=0)

So fix those issues (and hopefully also 30030) by using an unoccupied p2p_port.

The logic is similar to the extra_port() logic in the feature_bind_extra.py test.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33670.

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Code review ACK fa20275db32c5b9b0fe35effe2d1cf3d958e7310

ACK fa20275db32c5b9b0fe35effe2d1cf3d958e7310

LGTM, I also took a look recently trying to find the root cause but didn't come up with a better idea than just changing the port and hoping for the best.