tests: terminate subprocess in test-stdio.py in case of exception

If an error happened while reading the output of the subprocess, the pipe / TTY
buffer can fill up and prevent that the subprocess ends. Therefore we should
terminate the subprocess in case of an exception.

tests/test-stdio.py

@@ -99,6 +99,9 b' class TestStdio(unittest.TestCase):'
                 self.assertEqual(
                     _readall(stream_receiver, 1024), expected_output
                 )
+            except:  # re-raises
+                proc.terminate()
+                raise
             finally:
                 retcode = proc.wait()
             self.assertEqual(retcode, 0)