Fixed #26111 -- Clarified that fixtures are loaded once per TestCase.

Here's specifically what will happen:

* At the start of each test case, before ``setUp()`` is run, Django will

  flush the database, returning the database to the state it was in

  directly after :djadmin:`migrate` was called.

* At the start of each test, before ``setUp()`` is run, Django will flush the

  database, returning the database to the state it was in directly after

  :djadmin:`migrate` was called.

* Then, all the named fixtures are installed. In this example, Django will

  install any JSON fixture named ``mammals``, followed by any fixture named

  ``birds``. See the :djadmin:`loaddata` documentation for more

  details on defining and installing fixtures.

This flush/load procedure is repeated for each test in the test case, so you

can be certain that the outcome of a test will not be affected by another test,

or by the order of test execution.

For performance reasons, :class:`TestCase` loads fixtures once for the entire

test class, before :meth:`~TestCase.setUpTestData`, instead of before each

test, and it uses transactions to clean the database before each test. In any case,

you can be certain that the outcome of a test will not be affected by another

test or by the order of test execution.

By default, fixtures are only loaded into the ``default`` database. If you are

using multiple databases and set :attr:`multi_db=True