How to connect to remote PostgreSQL server
I suppose the simplest way is to let the tester use ssh to login to your server. Then the tester's execution environment is whatever your server exposes through the shell. (I presume that would include psql.)

As far as I know, all modern Unix derivatives install ssh by default. That would include Linux, Solaris, OS/X (Apple), and BSD. I believe that Windows does not ship with ssh, but Cygwin and PuTTY are commonly used to remedy that problem.