The original test would open a local port and then immediately close it
and use the port number in subsequent tests. Between the port being closed
and reused by the later process, it could be opened by some other program
on the machine.

Changed the test to run the server process directly and have it save the
assigned port to a text file to be used by client processes.

Fixes #5564.

LGTM=adg
R=golang-codereviews, gobot, adg
CC=golang-codereviews
https://golang.org/cl/72290043