Do not overwrite `output.write` in live tests.

The `write` method of the `output` object was being overwritten
in live tests, replacing it to a function that printed both to
the output and to stdout. This was not necessary, as it is
possible to use this function directly. Moreover, the function
code had two flaws:
  - It assumed that output was a particular (StringIO) class.
  This caused a problem during the migration (commit
  80bd30fc820d273d18d9c54566137301d9c40a4c).
  - It printed to stdout instead to stderr.
Using the function directly allows `output.write` to be used
inside it, preventing the first flaw. The second flaw has also
been solved.

product/ERP5Type/tests/ERP5TypeLiveTestCase.py

@@ -291,12 +291,11 @@ def runLiveTest(test_list, verbosity=1, stream=None, request_server_url=None, **
   if stream is None:
     output = StringIO()
   def print_and_write(data):
-    sys.stdout.write(data)
-    sys.stdout.flush()
-    return StringIO.write(output, data)
-  output.write = print_and_write
-  output.write("**Running Live Test:\n")
-  ZopeTestCase._print = output.write
+    sys.stderr.write(data)
+    sys.stderr.flush()
+    return output.write(data)
+  print_and_write("**Running Live Test:\n")
+  ZopeTestCase._print = print_and_write
 
   with warnings.catch_warnings():
     warnings.simplefilter(kw['warnings'])