tests: Add test for when instance member overrides class member.

tests/basics/class_instance_override.py

@@ -0,0 +1,10 @@
+# test that we can override a class method with an instance method
+
+class A:
+    def foo(self):
+        return 1
+
+a = A()
+print(a.foo())
+a.foo = lambda:2
+print(a.foo())