test: Fix broken fs_obj_fat fixture

test: Fix broken fs_obj_fat fixture

The removal of the size_gran parameter from mk_fs() leaves a stale
positional argument (1024) that is now interpreted as the fs_img
filename. Since 1024 is an integer, os.path.join() raises TypeError,
causing the fixture to silently skip via the bare except clause.

Drop the stale argument so the fixture works again.

Fixes: d030dc34d67a ("test: fs_helper: Drop the size_gran argument")
Signed-off-by: Simon Glass <sjg@chromium.org>