Check that _EXTENSION_PLUGIN contains all registered extensions

Author: radarhere

Tests/test_image.py

@@ -466,6 +466,9 @@ def test_registered_extensions_uninitialized(self) -> None:
         # Assert
         assert Image._initialized == 2
 
+        for extension in Image.EXTENSION:
+            assert extension in Image._EXTENSION_PLUGIN
+
     def test_registered_extensions(self) -> None:
         # Arrange
         # Open an image to trigger plugin registration