services: tor: Add a system test.

services: tor: Add a system test.

* gnu/services/networking.scm (tor-configuration->torrc): Set PidFile to
/var/run/tor/tor.pid in the base torrc configuration.
(tor-shepherd-service) <start>: Call make-forkexec-constructor/container with
a new #:pid-file argument to tell Shepherd where to find the PID file.  Add a
a new <file-system-mapping> to its existing #:mappings argument to share
/var/run/tor with the the container.
(tor-hidden-services-activation): Update docstring.  Create /var/run/tor and
set its permissions so only the tor user can access it.
* gnu/tests/networking.scm (%test-tor, %tor-os): New variables.
(run-tor-test): New procedure.