Refactor several unit tests to use SettingsStore directly. (#29744)

* Refactor notifications-test.ts

* Refactor other tests to stop mocking SettingsStore

test/unit-tests/components/views/settings/tabs/room/NotificationSettingsTab-test.tsx

@@ -37,6 +37,10 @@ describe("NotificatinSettingsTab", () => {
         NotificationSettingsTab.contextType = React.createContext<MatrixClient>(cli);
     });
 
+    afterEach(() => {
+        SettingsStore.reset();
+    });
+
     it("should prevent »Settings« link click from bubbling up to radio buttons", async () => {
         const tab = renderTab();