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

* Refactor notifications-test.ts

* Refactor other tests to stop mocking SettingsStore

test/unit-tests/components/structures/MatrixChat-test.tsx

@@ -484,6 +484,10 @@ describe("<MatrixChat />", () => {
                 );
             });
 
+            afterEach(() => {
+                SettingsStore.reset();
+            });
+
             it("should persist login credentials", async () => {
                 getComponent({ realQueryParams });