build: add a testclean target

This commit adds a target named testclean to allow for cleaning the
temporary files generated during a test run without having to use the
clean target.

PR-URL: https://github.com/nodejs/node/pull/29094


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>