Code review comment for ~raharper/curtin:fix/jenkins-filter-once-more

thanks.
For my feeble brain, I made the table below
http://paste.ubuntu.com/p/nXTNTksprF/

tests| ntfilters | result
 y | y | error
 n | n | use tests=( tests/vmtests )
 y | n | tests is set already, use "${tests[@]}"
 n | y | tests=$( $(vmtest-filter ...) )