tests: Update for jemalloc's option parsing

MALLOC_OPTIONS=J -> MALLOC_CONF=junk:true

PR: 287357
Reviewed by: markj
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50910

(cherry picked from commit 4f33d073003ccd91390709e14e5c1bf1f0c1c85c)