test: improve template value for test message

Include value that cause failure in error message in
test-cluster-master-kill.js.

PR-URL: https://github.com/nodejs/node/pull/16826


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>