Fix incorrect object in debounce test

I was trying to avoid having to import the Announcer in this test to keep things isolated, but it's so specific to the Announcer that I decided it didn't matter that much. Specifying the Announcer instance for the scope instead of generic thisArg logic simplified things quite a bit as well.

packages/wonder-blocks-announcer/src/__tests__/util/util.test.ts

@@ -20,8 +20,9 @@ describe("Debouncing messages", () => {
 
     test("resolving with the first argument passed if debounced multiple times", async () => {
         // ARRANGE
+        const announcer = Announcer.getInstance();
         const callback = jest.fn((message: string) => message);
-        const debounced = createDebounceFunction(window, callback, 500);
+        const debounced = createDebounceFunction(announcer, callback, 500);
 
         // ACT
         debounced("First message");