Explicitly disconnect in the 'exit' command

This fixes a race condition when running the tests. I think that the X11
server has more time to clean up the resources when we do an explicit
disconnect. The symptom I was seeing was that sometimes, i3 couldn’t
become the window manager on one of the Xdummy instances.

src/commands.c

@@ -1410,6 +1410,7 @@ void cmd_layout(I3_CMD, char *layout_str) {
  */
 void cmd_exit(I3_CMD) {
     LOG("Exiting due to user command.\n");
+    xcb_disconnect(conn);
     exit(0);
 
     /* unreached */