[Test Framework] Support setting the p2p/RPC port range over the cmd line

[fanquake]

"For supporting multiple tests running in parallel, I think it might make sense to add support to the test framework for specifying a range of ports to use for the p2p and rpc ports on the command line, which the rpc_tests.py script could use. The logic now of hoping that pid's don't collide modulo some big number is not sufficiently robust in my opinion."

Original comment from @sdaftuar here.

[maflcko]

#7972 includes a patch (ccccc59) to pass in a seed instead of relying on os.getpid(). Not sure if more than that is required/wanted.