test: only inspect on failure

The inspection was done in all cases so far and that's not necessary.
Therefore this changed this behavior to only inspect the input on
failure cases.

PR-URL: https://github.com/nodejs/node/pull/26360


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>