Add demo: Physics Tests 2D

Similar to its 3D counterpart, with tests using 2D physics.

2d/physics_tests/tests/functional/test_stack.gd

@@ -0,0 +1,39 @@
+extends Test
+
+
+export(int) var height = 10
+export(int) var width = 1
+export(Vector2) var box_size = Vector2(40.0, 40.0)
+export(Vector2) var box_spacing =  Vector2(0.0, 0.0)
+
+
+func _ready():
+	_create_stack()
+
+
+func _create_stack():
+	var root_node = $Stack
+
+	var template_body = create_rigidbody_box(box_size)
+
+	var pos_y = -0.5 * box_size.y - box_spacing.y
+
+	for level in height:
+		var row_node = Node2D.new()
+		row_node.position = Vector2(0.0, pos_y)
+		row_node.name = "Row%02d" % (level + 1)
+		root_node.add_child(row_node)
+
+		var pos_x = -0.5 * (width - 1) * (box_size.x + box_spacing.x)
+
+		for box_index in width:
+			var box = template_body.duplicate()
+			box.position = Vector2(pos_x, 0.0)
+			box.name = "Box%02d" % (box_index + 1)
+			row_node.add_child(box)
+
+			pos_x += box_size.x + box_spacing.x
+
+		pos_y -= box_size.y + box_spacing.y
+
+	template_body.queue_free()