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

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

Since commit dc7b3e56337ee9d8dcd8fe7d5cab71ef536d024f, '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.