functional tests: Remove portseed_offset from test runner

Summary:
PR description:

The portseed_offset is no longer needed in the test runner, since we already kill leftover processes

The pseudo-random portseed causes port collisions with other daemons. Core devs had issues with port 11211 (memcached), and we are seeing issues with 21325 (trezord).
This backport is a first step in avoiding random failures, by making the ports more deterministic again.

We cannot use the number of tests as a unique seed for Bitcoin ABC like Core does, because we run tests in parallel, so use the test number instead.

This is a backport of core#13421

Test Plan: for i in {1..4}; do ninja check-functional; done

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9759