Per-vdev ZAP tests must use $ZPOOL and $ZDB

Commit e0ab3ab introduced new per-vdev ZAP tests which should have
used the $ZPOOL and $ZDB variabled. The tests passed the automated
testing since both utilities but when running in-tree all of the new
tests fail.

Signed-off-by: Don Brady <don.brady@intel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4515