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.