client2: rendering: fix crash when moving a node

Previously, client2 crashed when an already rendered node was moved by
appending or inserting it twice within the same HTML tree.

    moving_node = Div('node 3')

    html = HTML(
        Div(
            'node 1',
            moving_node,
        ),
        Div('node 2'),
    )

    self.show(html)

    html[1].append(moving_node)

    self.show(html)  # resulted in: ERROR  lona.view_runtime  client error raised: node with id 952 is already cached

When a node is moved by adding it twice to the same node tree, internally, it
gets destroyed at the old location, and a new node, with the old id, gets
rendered in the new location.

Previously, the rendering engines node cache got not cleaned after the first
step, so all child nodes of `moving_node`, which is a text node with the text
"node 3" in this case, remained cached.
When the rendering engine attempted to render a new div, identical to the
former removed one, the cache function would raise an error because the old
text node still was cached.

This patch adds a cache cleaning call to the rendering engines `_remove_node`
method and adds a rendering test for moving nodes.

Signed-off-by: Florian Scherf <mail@florianscherf.de>

lona/client2/_lona/client2/rendering-engine.js

@@ -104,6 +104,7 @@ export class LonaRenderingEngine {
         node.remove();
 
         this._remove_widget_if_present(node_id);
+        this._clean_node_cache();
     };
 
     _remove_widget_if_present(node_id) {