This is the fourth patch in the series.
It removes any explicit port numbers in the test.
This may be useful for preventing some potential
future problems with parallel "make check".

Description from the commit message:

> Subject: [PATCH 4/4] tests: publish: don't bind the test server to a port.
>
> This way, multiple instances of 'make check TESTS=tests/publish.scm'
> can be run in parallel.  Also, there's no risk of the ports used
> in this test conflicting with ports assigned to system services
> anymore.  This also prevents any potential future conflicts
> with ports used by other tests that would lead to nondeterministic
> test failures when parallel tests are enabled.