test: replace `indexOf` with `includes`

PR-URL: https://github.com/nodejs/node/pull/13215
Refs: https://github.com/nodejs/node/issues/12586


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>