tests: Reduce boilerplate for users of 'system-test-runner'.
* gnu/tests/audio.scm, gnu/tests/base.scm, gnu/tests/ci.scm,
gnu/tests/cups.scm, gnu/tests/databases.scm, gnu/tests/desktop.scm,
gnu/tests/dict.scm, gnu/tests/docker.scm, gnu/tests/file-sharing.scm,
gnu/tests/ganeti.scm, gnu/tests/guix.scm, gnu/tests/ldap.scm,
gnu/tests/linux-modules.scm,
gnu/tests/mail.scm, gnu/tests/messaging.scm, gnu/tests/monitoring.scm,
gnu/tests/networking.scm, gnu/tests/nfs.scm,
gnu/tests/package-management.scm, gnu/tests/reconfigure.scm,
gnu/tests/rsync.scm, gnu/tests/security-token.scm,
gnu/tests/singularity.scm, gnu/tests/ssh.scm, gnu/tests/telephony.scm,
gnu/tests/version-control.scm, gnu/tests/virtualization.scm,
gnu/tests/web.scm: Remove (mkdir #$output) (chdir #$output) and
pass #$output as argument to 'system-test-runner'.
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