tests/basics: Use str.format instead of % for formatting messages.

Only use % formatting when testing % itself, because only str.format is
guaranteed to be available on any port.

tests/basics/class_notimpl.py

@@ -11,7 +11,7 @@ class C:
         self.value = value
 
     def __str__(self):
-        return "C(%s)" % self.value
+        return "C({})".format(self.value)
 
     def __add__(self, rhs):
         print(self, '+', rhs)