test: replace string concatenation with templates

PR-URL: https://github.com/nodejs/node/pull/15903


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Refael Ackermann <refack@gmail.com>