llext: test-simple: replace ARRAY_SIZE() with sizeof()

When identifying the size of an extension image we simply need its
size in bytes, not the number of array elements, that contain it.
Both happen to return the same value, because we use arrays of bytes,
but using sizeof() is clearer.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>