doc: moved test instructions to BUILDING.md

Fixes: https://github.com/nodejs/node/issues/23491

Duplicate test instructions were present in pull-requests.md
Merged the instructions in BUILDING.md and provided a link from
pull-requests.md

PR-URL: https://github.com/nodejs/node/pull/23949


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>