qemu-iotests: tests: do not set unused tmp variable

The previous commit removed the last usage of ${tmp} inside the tests
themselves; the only remaining users are sourced by check. So we can now
drop this variable from the tests.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: Bo Tu <tubo@linux.vnet.ibm.com>
Message-id: 1460472980-26319-4-git-send-email-silbe@linux.vnet.ibm.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
