tests/cpydiff/core_arguments: Move under Functions subsection.

This is the last "orphan" case.

tests/cpydiff/core_function_argcount.py

@@ -1,5 +1,5 @@
 """
-categories: Core
+categories: Core,Functions
 description: Error messages for methods may display unexpected argument counts
 cause: MicroPython counts "self" as an argument.
 workaround: Interpret error messages with the information above in mind.