code: function startClient(httpBackend, client) {
      httpBackend.when("GET", "/pushrules").respond(200, {});
      httpBackend.when("POST", "/filter").respond(200, { filter_id: "fid" });
      httpBackend.when("GET", "/sync").respond(200, INITIAL_SYNC_DATA);
  
      client.startClient();
  
      // set up a promise which will resolve once the client is initialised
      const deferred = Promise.defer();
      client.on("sync", function(state) {
          console.log("sync", state);
          if (state != "SYNCING") {
              return;
          }
          deferred.resolve();
      });
  
      return Promise.all([
          httpBackend.flushAllExpected(),
          deferred.promise,
      ]);
  }
docs: start the client, and wait for it to initialise