Add the settings needle that provides the 'workspace' tag back

Removing this broke the GNOME apps test entirely because there
was no GNOME 'workspace' needle any more.

I don't like this needle much, we should probably use
check_desktop_clean or something instead. But for now let's just
put it back.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

needles/gnome/apps/apps_settings_screen_black.json

@@ -0,0 +1,16 @@
+{
+  "properties": [],
+  "tags": [
+    "apps_settings_screen_black",
+    "workspace"
+  ],
+  "area": [
+    {
+      "xpos": 382,
+      "ypos": 280,
+      "width": 238,
+      "height": 171,
+      "type": "match"
+    }
+  ]
+}