test: use truncate in mk_fs()

While the dd command actually writes to the block device the truncate
command only updates the metadata (at least on ext4). This is faster and
reduces wear on the block device.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

test/py/tests/fs_helper.py

@@ -54,7 +54,7 @@ def mk_fs(config, fs_type, size, prefix, src_dir=None, size_gran = 0x100000):
 
     try:
         check_call(f'rm -f {fs_img}', shell=True)
-        check_call(f'dd if=/dev/zero of={fs_img} bs={size_gran} count={count}',
+        check_call(f'truncate -s $(( {size_gran} * {count} )) {fs_img}',
                    shell=True)
         check_call(f'mkfs.{fs_lnxtype} {mkfs_opt} {fs_img}', shell=True)
         if fs_type == 'ext4':