test: improve message in test-fs-readfile-pipe-large

Improve assertion message by including expected and actual values.

PR-URL: https://github.com/nodejs/node/pull/16840


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>