support/testing/tests/fs/test_ext: specify 16 MB as ext filesystem size

support/testing/tests/fs/test_ext: specify 16 MB as ext filesystem size

Since Qemu 5.1, the SD card size must be a power of two, so the
default size for ext2/3/4 filesystem images of 60 MB is not
suitable. Since 16 MB is used for the Ext4 test, let's use the same
value for the other tests as well (ext2, ext2r1, ext3). Without this
change, the ext2, ext2r1 and ext3 simply fail to run under Qemu >=
5.1.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>