support/testing: test_ubi: add image format on the qemu command line

Adding the Image format on the Qemu command line avoid this warning:

"WARNING: Image format was not specified for 'output/TestUbi/images/rootfs.ubi' and probing guessed raw.
         Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
         Specify the 'raw' format explicitly to remove the restrictions."

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>