test: Make PORT_MIN in test runner configurable #16918

This is needed when some ports in the port range are used by other processes. Note that simply assigning the ports dynamically does not work:

• We spin up several nodes per test (each node gets its own port)
• We run several tests in parallel

So to avoid nodes from different tests colliding on ports, the port assignment must be deterministic (can not be dynamic).

Fixes: #10869

Failing test on the freebsd cirrus-ci:

• https://cirrus-ci.com/task/5148751687057408?command=functional_test#L372

Passing test:

• https://cirrus-ci.com/task/6287511128637440

ACK fa69588537bc91c0aedbc89ef1760d89cbffad75 -- diff looks correct

ACK fa69588537bc91c0aedbc89ef1760d89cbffad75.

For reference https://cirrus-ci.org/guide/writing-tasks/#http-cache (from the source code)