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.
Fixed #23742 -- Added an option to reverse tests order.

This is useful for debugging side effects affecting tests that
are usually executed before a given test. Full suite and pair
tests sort cases more or less deterministically, thus some test
cross-dependencies are easier to reveal by reversing the order.

Thanks Preston Timmons for the review.
Fixed typo in docs/ref/django-admin.txt
Fixed #23890 -- Silenced numpy DeprecationWarnings in template tests.
Fixed #21587 -- Added a warning for changing default of RedirectView.permanent.
Fixed #23899 -- Added 'Generated by Django X.Y' to startproject template.
Fixed and restored assertions in OneToOneTests.test_foreign_key.

These assertions had been removed in 34ba86706f and 7fe554b2a3,
seemingly because they were referencing the wrong objects, and so
they started failing when the checking of object types (as well as
PK values) was introduced.
Fixed #23682 -- Enhanced circular redirects detection in tests.

When the test client detects a redirect to a URL seen in the
currently followed chain it will now raise a RedirectCycleError
instead of just returning the first repeated response.

It will also complain when a single chain of redirects is longer
than 20, as this often means a redirect loop with varying URLs,
and even if it's not actually one, such long chains are likely
to be treated as loops by browsers.

Thanks Preston Timmons, Berker Peksag, and Tim Graham for reviews.
Fixed bug in circular dependency algo for migration dependencies.

Previous algo only worked if the first item was a part of the loop,
and you would get an infinite loop otherwise (see test).

To fix this, it was much easier to do a pre-pass.

A bonus is that you now get an error message that actually helps you debug
the dependency problem.
Fixed flake8 warnings.
Fixed #23898 -- Added missing context to admin's deleted_selected view.

Thanks Redouane Zait for the report.
Added warning about get_inline_instances() permission checking; refs #23754.
Fixed #23754 -- Always allowed reference to the primary key in the admin

This change allows dynamically created inlines "Add related" button to work
correcly as long as their associated foreign key is pointing to the primary
key of the related model.

Thanks to amorce for the report, Julien Phalip for the initial patch,
and Collin Anderson for the review.
Fixed #23915 -- Made sure m2m fields through non-pk to_field are allowed in the admin.

refs #23754, #23862