ZTS: make uses of mktemp consistent

In all cases, rely on mktemp itself to make the best decision about
where to place the file or directory. In all cases, that decision will
be $TMPDIR, which we have set globally.

Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>