support/testing: test_ubi: reduce the rootfs.ubi size to 64M to match the Qemu...

support/testing: test_ubi: reduce the rootfs.ubi size to 64M to match the Qemu emulated flash device

The size of the cfi flash device emulated by Qemu is 64M not 128M [1].
Since Qemu >= 4.0, the size of the device must match the size of the block backend [2].

Fixes:

  qemu-system-arm: device requires 67108864 bytes, block backend provides 134217728 bytes

[1] https://git.qemu.org/?p=qemu.git;a=blob;f=hw/arm/vexpress.c;h=58481c07629aedb09864dcc72757ff7947e733bb;hb=f9baca549e44791be0dd98de15add3d8452a8af0#l50
[2] https://git.qemu.org/?p=qemu.git;a=commitdiff;h=06f1521795207359a395996c253c306f4ab7586e



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