Significantly speed up mkimg_test

It turns out that the majority of the test time for the mkimg tests isn't
mkimg itself but rather the use of jot and hexdump which can be quite slow
on emulated platforms such as QEMU.

On QEMU-RISC-V this reduces the time for kyua test mkimg_test from 655
seconds to 200. And for CheriBSD on QEMU-CHERI this saves 4-5 hours (25%
of the time for the entire testsuite!) since jot ends up triggering slow
functions inside the QEMU emulation a lot.

Reviewed By: lwhsu
Differential Revision: https://reviews.freebsd.org/D26796