tests/cpydiff/core_function_unpacking: Fill in workaround.

tests/cpydiff/core_function_unpacking.py

 """
 categories: Core,Functions
-description: Unpacking function arguments in non-last position gives incorrect result
+description: Unpacking function arguments in non-last position isn't detected as an error
 cause: Unknown
-workaround: Unknown
+workaround: The syntax below is invalid, never use it in applications.
 """
 print(*(1, 2), 3)