Add a test with invalid unicode in spawned stdout.

stderr features identical behaviour, so is currently untested.

tests/unit/misc/test_guiprocess.py

@@ -63,6 +63,20 @@ def test_start(proc, qtbot, message_mock, py_proc):
     assert qutescheme.spawn_output == proc._spawn_format(stdout="test")
 
 
+def test_stdout_malformed_utf8(proc, qtbot, message_mock, py_proc):
+    """Test simply starting a process."""
+    with qtbot.waitSignals([proc.started, proc.finished], timeout=10000,
+                           order='strict'):
+        argv = py_proc(r"""
+            import sys
+            sys.stdout.buffer.write(b"A\x80B")
+            sys.exit(0)
+            """)
+        proc.start(*argv)
+    assert not message_mock.messages
+    assert qutescheme.spawn_output == proc._spawn_format(stdout="A\ufffdB")
+
+
 def test_start_verbose(proc, qtbot, message_mock, py_proc):
     """Test starting a process verbosely."""
     proc.verbose = True