Prep for matrix-js-sdk supporting intentional mentions. (#10431)

Add a method to the fake MatrixClient used in tests for compatibility with
changes in matrix-org/matrix-js-sdk#3092. This stops the downstream tests
from failing on that PR.

test/test-utils/test-utils.ts

@@ -176,6 +176,7 @@ export function createTestClient(): MatrixClient {
         isUserIgnored: jest.fn().mockReturnValue(false),
         getCapabilities: jest.fn().mockResolvedValue({}),
         supportsThreads: () => false,
+        supportsIntentionalMentions: () => false,
         getRoomUpgradeHistory: jest.fn().mockReturnValue([]),
         getOpenIdToken: jest.fn().mockResolvedValue(undefined),
         registerWithIdentityServer: jest.fn().mockResolvedValue({}),