tests: Add test for "not" of a user defined class.

tests/basics/unary_op.py

@@ -15,3 +15,13 @@ print(not [])
 print(not [1,])
 print(not {})
 print(not {1:1})
+
+# check user instance
+class A: pass
+print(not A())
+
+# check user instances derived from builtins
+class B(int): pass
+print(not B())
+class C(list): pass
+print(not C())