tests/servers.pm: pick unused port number with a server socket

tests/servers.pm: pick unused port number with a server socket

This change replaces the previous method of picking a port number at
random to try to start servers on, then retrying up to ten times with
new random numbers each time, with a function that creates a server
socket on port zero, thereby getting a suitable random port set by the
kernel. That server socket is then closed and that port number is used
to setup the actual test server on.

There is a risk that *another* server can be started on the machine in
the time gap, but the server verification feature will detect that.

Closes #11220