doc: revise intro sentence for assert

Simplify language, remove redundancy ("tests that can be used to
test") and remove wordy passive voice ("that can be used to" replaced
with "for").

PR-URL: https://github.com/nodejs/node/pull/28226


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>