fix: update testcase

Author: hackape

packages/playground/src/App.svelte

@@ -1,7 +1,7 @@
 <script>
-    import Counter from "./lib/Counter.svelte";
+	export let foo;
+	let bar = 'baz';
+	foo.bar = bar;
 </script>
 
-<div>
-    Hello world!
-</div>
+child: {typeof foo}

packages/svelte/test/runtime/samples/binding-backflow/Child.svelte

@@ -6,6 +6,10 @@
     value = { foo: 'kid' }
   }
 
+  if (testcase === 'init_mutate') {
+    value.foo = 'kid'
+  }
+
   $: if (testcase === 'reactive_update') {
     value = { foo: 'kid' }
   }

packages/svelte/test/runtime/samples/binding-backflow/_config.js

@@ -6,7 +6,8 @@ export default {
 				{ testcase: 'child_default_populate_parent', value: undefined },
 				{ testcase: 'reactive_update', value: { foo: 'mon' } },
 				{ testcase: 'reactive_mutate', value: { foo: 'mon' } },
-				{ testcase: 'init_update', value: { foo: 'mon' } }
+				{ testcase: 'init_update', value: { foo: 'mon' } },
+				{ testcase: 'init_mutate', value: { foo: 'mon' } }
 			]
 		};
 	},
@@ -36,5 +37,9 @@ export default {
 		p = parents['init_update'];
 		assert.deepEqual(p.value, { foo: 'kid' });
 		assert.equal(p.updates.length, 2);
+
+		p = parents['init_mutate'];
+		assert.deepEqual(p.value, { foo: 'kid' });
+		assert.equal(p.updates.length, 2);
 	}
 };