A malformed /etc/sudoers.d/left4me would lock sudo on the target
(blast radius: every other bundle using sudo at apply time). bw's
file: items support test_with, which runs the supplied command on the
locally-rendered file before transfer. Use it to gate the sudoers
file on visudo -cf — analogous to the visudo -cf check the original
deploy script ran inline (deploy-test-server.sh:186).