Fix ThreadView tests not using thread flag (#9547)

Fixes tests failing from changes made by https://github.com/matrix-org/matrix-js-sdk/pull/2856

test/Notifier-test.ts

@@ -81,6 +81,7 @@ describe("Notifier", () => {
             decryptEventIfNeeded: jest.fn(),
             getRoom: jest.fn(),
             getPushActionsForEvent: jest.fn(),
+            supportsExperimentalThreads: jest.fn().mockReturnValue(false),
         });
 
         mockClient.pushRules = {

test/components/structures/ThreadView-test.tsx

@@ -108,6 +108,7 @@ describe("ThreadView", () => {
         stubClient();
         mockPlatformPeg();
         mockClient = mocked(MatrixClientPeg.get());
+        jest.spyOn(mockClient, "supportsExperimentalThreads").mockReturnValue(true);
 
         room = new Room(ROOM_ID, mockClient, mockClient.getUserId() ?? "", {
             pendingEventOrdering: PendingEventOrdering.Detached,