doc: add info how to run single tests to BUILDING.md

The information on how to run single tests was missing in
BUILDING.md.

PR-URL: https://github.com/nodejs/node/pull/23490
Fixes: https://github.com/nodejs/node/issues/23491


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>