Add failing test for gh-6025

IPython/core/tests/test_inputtransformer.py

@@ -212,6 +212,10 b' syntax_ml = \\'
           ('...', ''),
           ('...     return x', '    return x'),
           ],
+         [('board = """....', 'board = """....'),
+          ('....', '....'),
+          ('...."""', '...."""'),
+          ],
         ],
 
        ipy_prompt =