ZTS: clean up leftover ibackup_trunc files

zfs_receive_raw_incremental did not clean up ibackup_trunc.* files
left over from running the test.

Also changing the path of the ibackup files so they can be placed
in the correct directories when /var/tmp is not the temporary
directory.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: bunder2015 <omfgbunder@gmail.com>
Closes #7430