Add stub for getVisibleRooms()

Fixes the tests

test/components/views/rooms/RoomList-test.js

@@ -94,6 +94,7 @@ describe('RoomList', () => {
             createRoom({tags: {'m.lowpriority': {}}, name: 'Some unimportant room'}),
             createRoom({tags: {'custom.tag': {}}, name: 'Some room customly tagged'}),
         ];
+        client.getVisibleRooms = client.getRooms;
 
         const roomMap = {};
         client.getRooms().forEach((r) => {

test/test-utils.js

@@ -74,6 +74,7 @@ export function createTestClient() {
         getPushActionsForEvent: sinon.stub(),
         getRoom: sinon.stub().returns(mkStubRoom()),
         getRooms: sinon.stub().returns([]),
+        getVisibleRooms: sinon.stub().returns([]),
         getGroups: sinon.stub().returns([]),
         loginFlows: sinon.stub(),
         on: sinon.stub(),