Add a new unit test to ensure incremental decoding works

Author: scootermon

tests/runners.py

@@ -539,6 +539,16 @@ def writes_and_flushes_to_stderr(self):
             err.write.assert_called_once_with("whatever")
             err.flush.assert_called_once_with()
 
+        def handles_incremental_decoding(self):
+            # 𠜎 is 4 bytes in utf-8
+            expected_out = "𠜎" * 50
+            runner = self._runner(out=expected_out)
+            # Make sure every read returns a partial character.
+            runner.read_chunk_size = 3
+            out = StringIO()
+            runner.run(_, out_stream=out)
+            assert out.getvalue() == expected_out
+
     class input_stream_handling:
         # NOTE: actual autoresponder tests are elsewhere. These just test that
         # stdin works normally & can be overridden.