ZTS: Replace /var/tmp with $TEST_BASE_DIR

Remove a few hardcoded instances of /var/tmp. This should use
the $TEST_BASE_DIR in order to allow the ZTS to be optionally
run in an alternate directory using zfs-tests.sh -d <path>.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9775