test: do not fail SLOW tests if they are not slow

PR-URL: https://github.com/nodejs/node/pull/25868
Refs: https://github.com/nodejs/node/issues/25867


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gus Caplan <me@gus.host>