Add initial batch of Dialog tests

src/lib/components/switch/_ManagedSwitch.svelte

@@ -3,9 +3,14 @@
 
   // This component is only for use in tests
   export let initialChecked = false;
+  export let onChange = () => {};
   let state = initialChecked;
 </script>
 
-<Switch checked={state} on:change={(e) => (state = e.detail)} on:change>
+<Switch
+  checked={state}
+  on:change={(e) => (state = e.detail)}
+  on:change={onChange}
+>
   <slot />
 </Switch>