test: make common.PIPE process unique

* includes a tiny bit of refactoring in adjacent lines.
* fixes 1 test and 1 benchmark that depended on PIPE being constant.

PR-URL: https://github.com/nodejs/node/pull/14168
Fixes: https://github.com/nodejs/node/issues/14128


Reviewed-By: James M Snell <jasnell@gmail.com>