REGTESTS: specify explicit chroot in confs using unix-stream servers

REGTESTS: specify explicit chroot in confs using unix-stream servers

For convenience reasons, a small tens of tests make use of unix sockets
on server lines, and as such cannot be chrooted. Given that non-chrooted
startups are frowned upon and even emit a warning for privileged users,
let's explicitly use the recommended "chroot /" for them.