build: fix broken build (#38274)

```
export const __core_private_testing_placeholder__ = '';
```
This API should be removed. But doing so seems to break `google3` and
so it requires a bit of investigation. A work around is to mark it as
`@codeGenApi` for now and investigate later.

PR Close #38274

goldens/public-api/core/testing/testing.d.ts

@@ -1,3 +1,6 @@
+/** @codeGenApi */
+export declare const __core_private_testing_placeholder__ = "";
+
 export declare function async(fn: Function): (done: any) => any;
 
 export declare class ComponentFixture<T> {

packages/core/testing/src/before_each.ts

@@ -27,6 +27,13 @@ if (_global.beforeEach) {
   });
 }
 
-// TODO(juliemr): remove this, only used because we need to export something to have compilation
-// work.
+/**
+ * This API should be removed. But doing so seems to break `google3` and so it requires a bit of
+ * investigation.
+ *
+ * A work around is to mark it as `@codeGenApi` for now and investigate later.
+ *
+ * @codeGenApi
+ */
+// TODO(iminar): Remove this code in a safe way.
 export const __core_private_testing_placeholder__ = '';