test: add relative path to accommodate limit

Found that libuv had a path character limit of 108.

Used path.relative() to set prefix with relative path.

Also added comments explaining the use of a relative path.

PR-URL: https://github.com/nodejs/node/pull/12601


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>