ZTS: fix flakiness in cp_files_002_pos

Fix RANDOM to not return zero.

Overwriting with dd ... count=0 does not test anything.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Robert Evans <evansr@google.com>
Closes #16029