fix parallel execution of swapon tests

And apparantly the CI env already has an md device so be relaxed about which md device is added.

PR: 186252
Reported by: markj
Fixes: 1ee33e3a578c35ff0e8ef5db87a8005f1f99546a "swapon: parse md.eli device for encrypted swapfile from fstab"
Pull Request: https://github.com/freebsd/freebsd-src/pull/1747

(cherry picked from commit 83b08f40264177ecf03fce808474815816f9846f)