utils: make git object type of TestObject obvious

In debugging some problems with MissingObjectFinder, I've repeatedly
gotten confused by some error messages that talk about "TestObject has
no attribute X" for varying X. Poking a clearer name into __name__ for
TestObject helps clarify what we're really seeing.

dulwich/tests/utils.py

@@ -102,6 +102,7 @@ def make_object(cls, **attrs):
         __dict__ instead of __slots__.
         """
         pass
+    TestObject.__name__ = 'TestObject_' + cls.__name__
 
     obj = TestObject()
     for name, value in attrs.items():