tests/cpydiff: Add CPy diff test for assignment expression behaviour.

tests/cpydiff/syntax_assign_expr.py

@@ -0,0 +1,7 @@
+"""
+categories: Syntax,Operators
+description: MicroPython allows using := to assign to the variable of a comprehension, CPython raises a SyntaxError.
+cause: MicroPython is optimised for code size and doesn't check this case.
+workaround: Do not rely on this behaviour if writing CPython compatible code.
+"""
+print([i := -1 for i in range(4)])