tools: exclude benchmark code from coverage report

Refs: https://github.com/nodejs/build/issues/1676#issuecomment-459218651

PR-URL: https://github.com/nodejs/node/pull/25841


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>