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.
37 jobs for for_testrunner_2 in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner2

02:18:27

failed ERP5.UnitTest-TestRunner2

04:12:23

passed ERP5.CodingStyleTest-TestRunner2

02:12:01

passed ERP5.CodingStyleTest-TestRunner2

01:43:32

passed ERP5.CodingStyleTest-TestRunner2

01:17:57

passed ERP5.CodingStyleTest-TestRunner2

01:13:02

passed ERP5.CodingStyleTest-TestRunner2

01:22:17

passed ERP5.CodingStyleTest-TestRunner2

01:18:58

passed ERP5.CodingStyleTest-TestRunner2

01:57:23

passed ERP5.CodingStyleTest-TestRunner2

01:54:27

passed ERP5.CodingStyleTest-TestRunner2

01:31:37

passed ERP5.CodingStyleTest-TestRunner2

02:03:56

passed ERP5.CodingStyleTest-TestRunner2

01:52:09

passed ERP5.CodingStyleTest-TestRunner2

01:03:11

passed ERP5.CodingStyleTest-TestRunner2

01:09:30

passed ERP5.CodingStyleTest-TestRunner2

01:22:14

passed ERP5.CodingStyleTest-TestRunner2

02:20:36

passed ERP5.CodingStyleTest-TestRunner2

07:29:23

passed ERP5.CodingStyleTest-TestRunner2

01:47:12

passed ERP5.CodingStyleTest-TestRunner2

01:35:22

passed ERP5.UnitTest-TestRunner2

03:55:57

passed ERP5.UnitTest-TestRunner2

02:24:02

passed ERP5.UnitTest-TestRunner2

01:52:31

failed ERP5.UnitTest-TestRunner2

03:06:25

passed ERP5.UnitTest-TestRunner2

02:00:51

failed ERP5.UnitTest-TestRunner2

02:53:24

passed ERP5.UnitTest-TestRunner2

02:02:13

passed ERP5.UnitTest-TestRunner2

01:58:14

passed ERP5.UnitTest-TestRunner2

02:15:49

passed ERP5.UnitTest-TestRunner2

02:26:06

passed ERP5.UnitTest-TestRunner2

02:22:40

failed ERP5.UnitTest-TestRunner2

01:59:23

passed ERP5.UnitTest-TestRunner2

03:31:04

failed ERP5.UnitTest-TestRunner2

09:05:19

passed ERP5.UnitTest-TestRunner2

02:03:48

failed ERP5.UnitTest-TestRunner2

02:19:25

passed ERP5.UnitTest-TestRunner2

03:04:55