tests/cpydiff: Add test of underscore-in-literals.

Signed-off-by: Jeff Epler <jepler@gmail.com>

tests/cpydiff/syntax_literal_underscore.py

@@ -0,0 +1,19 @@
+"""
+categories: Syntax,Literals
+description: MicroPython accepts underscores in numeric literals where CPython doesn't
+cause: Different parser implementation
+
+MicroPython's tokenizer ignores underscores in numeric literals, while CPython
+rejects multiple consecutive underscores and underscores after the last digit.
+
+workaround: Remove the underscores not accepted by CPython.
+"""
+
+try:
+    print(eval("1__1"))
+except SyntaxError:
+    print("Should not work")
+try:
+    print(eval("1_"))
+except SyntaxError:
+    print("Should not work")