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 80bd30fc).
• 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.