Name testing environmen variable NETWORK_DISPLAYS_DUMMY

This was renamed to ND_DUMMY together with other renames, but it seems
more sensible to have a longer string in this case.

README.md

@@ -18,7 +18,7 @@ Mutter currently has an issue when streaming from a multi-monitor setup. See
 Testing
 =======
 
-For testing purposes you can run with ND_DUMMY=1 set. In that case, a dummy
+For testing purposes you can run with NETWORK_DISPLAYS_DUMMY=1 set. In that case, a dummy
 sink will be provided that allows connecting on localhost using any RTSP capable
 client to test WFD streaming.
 

src/nd-window.c

@@ -340,7 +340,7 @@ gnome_nd_window_init (NdWindow *self)
   self->wfd_p2p_registry = nd_wfd_p2p_registry_new (self->meta_provider);
   nd_sink_list_set_provider (self->find_sink_list, ND_PROVIDER (self->meta_provider));
 
-  if (g_strcmp0 (g_getenv ("ND_DUMMY"), "1") == 0)
+  if (g_strcmp0 (g_getenv ("NETWORK_DISPLAYS_DUMMY"), "1") == 0)
     {
       g_autoptr(NdDummyProvider) dummy_provider = NULL;