PLT-5109/PLT-5112 Fixed commented out unit tests (#4951)

* Removed note about unconfirmed race condition

* Fixed unit tests in client_channel.test.jsx

* Fixed updateActive unit test on client

webapp/actions/user_actions.jsx

@@ -136,7 +136,6 @@ function populateDMChannelsWithProfiles(userIds) {
     const currentUserId = UserStore.getCurrentId();
 
     for (let i = 0; i < userIds.length; i++) {
-        // TODO There's a race condition here for DM channels if those channels aren't loaded yet
         const channelName = getDirectChannelName(currentUserId, userIds[i]);
         const channel = ChannelStore.getByName(channelName);
         if (channel) {