Refactor image processor testers (#25450)

* Refactor image processor test mixin

- Move test_call_numpy, test_call_pytorch, test_call_pil to mixin
- Rename mixin to reflect handling of logic more than saving
- Add prepare_image_inputs, expected_image_outputs for tests

* Fix for oneformer

tests/models/mgp_str/test_processor_mgp_str.py

@@ -46,7 +46,7 @@ class MgpstrProcessorTest(unittest.TestCase):
 
     @property
     def image_processor_dict(self):
-        return self.image_processor_tester.prepare_image_processor_dict()
+        return self.prepare_image_processor_dict()
 
     def setUp(self):
         self.image_size = (3, 32, 128)