Check error message

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: brijkapadia

Lib/test/test_decimal.py

@@ -4153,7 +4153,7 @@ def test_signaldict_repr(self):
         ctx = Context(prec=7)
         mapping = ctx.flags
         del ctx
-        with self.assertRaises(ValueError):
+        with self.assertRaisesRegex(ValueError, 'invalid signal dict'):
             repr(mapping)
 
 class PyContextFlags(ContextFlags, unittest.TestCase):