mod(container): make reposition public and split from resize

src/testing.zig

@@ -121,7 +121,8 @@ pub fn expectContainerScreen(size: Point, container: *Container(event.SystemEven
     defer renderer.deinit();
 
     try renderer.resize(size);
-    container.resize(.{}, size);
+    container.resize(size);
+    container.reposition(.{});
     try renderer.render(Container(event.SystemEvent), container);
 
     try expectEqualCells(.{}, renderer.size, expected, renderer.screen);