tests/cpydiff: Add test file for annotation expressions.

Document the behavior on expression annotations rejected by CPython but
accepted by MicroPython.

Closes issue micropython#19031.

Signed-off-by: Jeff Epler <jepler@unpythonic.net>

Author: jepler

tests/cpydiff/syntax_annotation_expression.py

@@ -0,0 +1,24 @@
+"""
+categories: Syntax,Annotations
+description: MicroPython accepts type annotations on expressions where CPython forbids them.
+cause: To reduce code size, MicroPython does not check the form of expressions with type annotations applied.
+workaround: Always check for valid Python code using a linting tool.
+
+The expressions themselves are not evaluated.
+"""
+
+
+def test(expr):
+    code = f"def f():\n    {expr}: int"
+    print(code)
+    try:
+        exec(code)
+        print("OK")
+    except SyntaxError as e:
+        print("SyntaxError")
+    print()
+
+
+test("print('test')")
+test("[x,y]")
+test("x,y")