Updated testing documentation following 498ae3a36069e2d

- commit/rollback are no longer replaced by nop
- the warning about not using TestCase when testing transactional
  behavior belongs to TestCase section, not TransactionTestCase
Fixed #13694 -- Made SafeMIMEText's constructor compatible with MIMEText.
Removed workaround for lack of os.getpid() in Jython.

The Jython bug was fixed in http://bugs.jython.org/issue1518
(tested on Jython 2.7b3); also updated make_msgid() to be more like
the version in Python 3.2+; refs #23905.

Thanks Simon Charette for testing and review.
Fixed #23888 -- Fixed crash in File.__repr__() when name contains unicode.
Fixed #23907 -- Added admin.site.register in tutorial 2 for consistency.
Changed test_runner imports to name objects imported from test.runner.
Improved line breaking in runtests docs.