ZTS: Change cp_stress to fit timings (#16369)

cp_stress is getting killed on the new QEMU-based github runners
we're developing. The problem is that the Linux based runners
should do 10 RUNS, where the FreeBSD based runners only have 3
RUNS to succeed.

This patch removes this different handling of Linux and FreeBSD.
The cp_stress test is running fine in around 2 minutes now.

Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>