ZTS: replace uses of TMPDIR with mktemp

Most of these are trying to use TMPDIR to put their work files somewhere
sensible. Now that we've set up correctly, they can all just use mktemp
to do the job.

In a couple of places cleaning up temp files wasn't being done
correctly, which has been fixed.

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>