ZTS: zfs-tests: use configured FILEDIR for all temp paths

The default file vdevs, constrained binpath and temporary runfiles were
all explicitly places in /var/tmp. Instead, put them under FILEDIR,
which is set from -d and defaults to /var/tmp. TEST_BASE_DIR is also
initialised from FILEDIR, which means all data for the run will now end
up under the operator-specified data dir.

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>