tests: Introduce 'simple-operating-system' and use it.

* gnu/tests.scm (%simple-os): New macro.
(simple-operating-system): New macro.
* gnu/tests/base.scm (%simple-os): Define using 'simple-operating-system'.
(%mcron-os): Use 'simple-operating-system'.
* gnu/tests/mail.scm (%opensmtpd-os): Likewise.
* gnu/tests/messaging.scm (%base-os, os-with-service): Remove.
(run-xmpp-test): Use 'simple-operating-system'.
* gnu/tests/networking.scm (%inetd-os): Likewise.
* gnu/tests/ssh.scm (%base-os, os-with-service): Remove.
(run-ssh-test): Use 'simple-operating-system'.
* gnu/tests/web.scm (%nginx-os): Likewise.