efi_selftest: partition label for test image

efi_selftest_disk_image.h contains a disk image. We use it to test the
EFI_FILE_PROTOCOL. The patch sets the partition label to 'U-BOOT TEST'.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>