doc: change error message testing policy

Dynamic error messages often contain important information that
depends on the context, e.g., which argument caused an
ERR_INVALID_ARG_TYPE, which type was expected, and which type was
received. I don't think that internal breakage when changing a dynamic
error message should prevent us from testing such properties properly.

PR-URL: https://github.com/nodejs/node/pull/31421


Reviewed-By: Rich Trott <rtrott@gmail.com>