tests: Add rudimentary color device test

Tests that test case EDID is setup correctly, and that color devices for
monitors are created.

tests/color: Add hotplugging tests

Checks that changing the number of connected monitors reflects the
number of current color devices, and that we end up with the correct end
state.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2164>

src/backends/meta-color-manager.c

@@ -380,3 +380,12 @@ meta_color_manager_is_ready (MetaColorManager *color_manager)
 
   return priv->is_ready;
 }
+
+int
+meta_color_manager_get_num_color_devices (MetaColorManager *color_manager)
+{
+  MetaColorManagerPrivate *priv =
+    meta_color_manager_get_instance_private (color_manager);
+
+  return g_hash_table_size (priv->devices);
+}