Fix printing out error message when test fails and run with -bb

Lib/test/test_signal.py

@@ -473,7 +473,7 @@ class SiginterruptTest(unittest.TestCase):
                 stdout = first_line + stdout
                 exitcode = process.wait()
                 if exitcode not in (2, 3):
-                    raise Exception("Child error (exit code %s): %s"
+                    raise Exception("Child error (exit code %s): %r"
                                     % (exitcode, stdout))
                 return (exitcode == 3)