tests: a fix for ARC and MPU VER 3

For ARC MPU version 3, the defined partitions are not added to MPU
when appmem_init_app_memory is doning app_bss_zero().

So need to disable mpu first to allow appmem_init_app_memory to
access all partitions.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>