test: fix child-process-pipe-dataflow

Make sure all the `wc` process stdout data is received before checking
its validity.

Fixes: https://github.com/nodejs/node/issues/25988

PR-URL: https://github.com/nodejs/node/pull/36366


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>