test: Include /sbin to the PATH when creating ext4 disk image

On some distributions the mkfs.ext4 is under /sbin and /sbin is not set
for mere users. Include /sbin to the PATH when creating ext4 disk image,
so that users won't get a scary traceback from Python.

Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>