tests: Use 'virtual-machine' records instead of monadic procedures.

tests: Use 'virtual-machine' records instead of monadic procedures.

* gnu/tests/base.scm (%test-basic-os): Use 'let*' instead of 'mlet*' and
'virtual-machine' instead of 'system-qemu-image/shared-store-script'.
(run-mcron-test): Likewise.
(run-nss-mdns-test): Likewise.
* gnu/tests/dict.scm (run-dicod-test): Likewise.
* gnu/tests/mail.scm (run-opensmtpd-test): Likewise.
(run-exim-test): Likewise.
* gnu/tests/messaging.scm (run-xmpp-test): Likewise.
* gnu/tests/networking.scm (run-inetd-test): Likewise.
* gnu/tests/nfs.scm (run-nfs-test): Likewise.
* gnu/tests/ssh.scm (run-ssh-test): Likewise.
* gnu/tests/web.scm (run-nginx-test): Likewise.