tests: Honor the return value of 'start-service'.

Since commit dc7b3e5633, 'start-service' returns
the Shepherd's representation of the service as a sexp, and '#f' if the
service fails to start.  Also, it doesn't throw an exception when the service
fails to start, so relying on an exception instead of relying on its return
value is a false positive.

* gnu/tests/base.scm (run-nss-mdns-test): Split tests so to check the return
value of 'start-service'.
(run-mcron-test, run-nss-mdns-test): Use 'test-assert' with the return value
of 'start-service'.
* gnu/tests/admin.scm (run-tailon-test): Idem.
* gnu/tests/dict.scm (run-dicod-test): Idem.
* gnu/tests/mail.scm (run-opensmtpd-test, run-exim-test, run-dovecot-test):
Idem.
* gnu/tests/messaging.scm (run-xmpp-test, run-bitlbee-test): Idem.
* gnu/tests/nfs.scm (run-nfs-test): Idem.
* gnu/tests/rsync.scm (run-rsync-test): Idem.
* gnu/tests/ssh.scm (run-ssh-test): Idem.
* gnu/tests/version-control.scm (run-cgit-test, run-git-http-test): Idem.
* gnu/tests/web.scm (run-php-fpm-test): Idem.

gnu/tests/messaging.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2017 Clément Lassieur <clement@lassieur.org>
+;;; Copyright © 2017, 2018 Clément Lassieur <clement@lassieur.org>
 ;;; Copyright © 2017, 2018 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -102,13 +102,11 @@
           (test-begin "xmpp")
 
           ;; Wait for XMPP service to be up and running.
-          (test-eq "service running"
-            'running!
+          (test-assert "service running"
             (marionette-eval
              '(begin
                 (use-modules (gnu services herd))
-                (start-service 'xmpp-daemon)
-                'running!)
+                (start-service 'xmpp-daemon))
              marionette))
 
           ;; Check XMPP service's PID.
@@ -196,13 +194,11 @@
 
           (test-begin "bitlbee")
 
-          (test-eq "service started"
-            'running!
+          (test-assert "service started"
             (marionette-eval
              '(begin
                 (use-modules (gnu services herd))
-                (start-service 'bitlbee)
-                'running!)
+                (start-service 'bitlbee))
              marionette))
 
           (test-equal "valid PID"