ZTS: replace all uses of /var/tmp with TEST_BASE_DIR

The operator can override TEST_BASE_DIR by setting its source var
FILEDIR through zfs-tests.sh -d. There were a handful of cases where
this was not honoured.

By default FILEDIR (and so TEST_BASE_DIR) is /var/tmp, so there should
be no functional change if the operator does nothing.

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>