patch 7.4.1484
Problem:    Channel "err-io" value "out" is not supported.
Solution:   Connect stderr to stdout if wanted.
diff --git a/src/testdir/test_channel_pipe.py b/src/testdir/test_channel_pipe.py
index 5994d27..2097d3e 100644
--- a/src/testdir/test_channel_pipe.py
+++ b/src/testdir/test_channel_pipe.py
@@ -18,9 +18,12 @@
             print("Goodbye!")
             sys.stdout.flush()
             break
-        if typed.startswith("echo"):
+        if typed.startswith("echo "):
             print(typed[5:-1])
             sys.stdout.flush()
+        if typed.startswith("echoerr"):
+            print(typed[8:-1], file=sys.stderr)
+            sys.stderr.flush()
         if typed.startswith("double"):
             print(typed[7:-1] + "\nAND " + typed[7:-1])
             sys.stdout.flush()