Dispatch slotchange event

The first time a `slotchange` event is registered, we setup a SlotChangeMonitor
on the page. This uses a global (ugh) MutationEvent to detect slot changes.

We could improve the perfomance of this by installing a MutationEvent per
custom element, but a global is obviously a lot easier.

Our MutationEvent currently fired _during_ the changes. This is problematic
(in general, but specifically for slotchange). You can image something like:

```
slot.addEventListener('slotchange', () => {
   // do something with slot.assignedNodes()
});
```

But, if we dispatch the `slotchange` during the MutationEvent, assignedNodes
will return old nodes. So, our SlotChangeMonitor uses the page scheduler to
schedule dispatches on the next tick.

src/tests/testing.js

@@ -107,9 +107,11 @@
     }
     testing._status = 'ok';
 
-    const script_id = testing._captured?.script_id || document.currentScript.id;
-    testing._executed_scripts.add(script_id);
-    _registerErrorCallback();
+    if (testing._captured || document.currentScript) {
+    	const script_id = testing._captured?.script_id || document.currentScript.id;
+    	testing._executed_scripts.add(script_id);
+	    _registerErrorCallback();
+    }
   }
 
   // We want to attach an onError callback to each <script>, so that we can