ZTS: zpool_split_indirect deletes zfstest log file

ZTS: zpool_split_indirect deletes zfstest log file

The cleanup routine for this test attempts to remove some temporary
files with `rm -f $VDEV_*`, but VDEV_ is undefined. As a result, all
files in the current working directory (/var/tmp/test_results/current)
get removed instead. This includes the complete log file of all tests.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: John Kennedy <john.kennedy@delphix.com>
Closes #10324