Where can I find documentation for the WP_UnitTestCase factory classes?

As far as I know, there is no documentation for it at present. The official source is here.

I have also written a tutorial on unit testing WordPress plugins, which gives some detail about this feature.

One of the advantages of using WP_UnitTestCase is its factories.
These can be accessed through the factory member variable. The
factory is an object with properties that are each an instance of
one of the classes defined in includes/factory.php.
What do they do, you ask? They make it very simple to create users,
posts, terms, etc., wherever you need them in your test. So, instead
of doing this:

$args = array( /* A bunch of user data you had to make up */ );
wp_insert_user( $args );

You can just do this:

$user_id = $this->factory->user->create();

But wait, it gets even better. What if you need many users
(or posts, or whatever)? You can just create them in bulk like this:

$user_ids = $this->factory->user->create_many( 25 );

That will create 25 users that you can use in your test.

The factory has the following properties that you can use:

• $post
• $attachment
• $comment
• $user
• $term
• $category
• $tag
• $blog

They may all be used in the same manner as demonstrated in the above
example with the $user factory. For example, you can create a post
like this:

$this->factory->post->create();

You can also specify particular arguments to use for creating the
object. In the above example we created a post, but it wasn’t assigned
to a particular user (the post_author field will default to
0). Sometimes we may want the post assigned to a user instead.
We’d do that like this:

$user_id = $this->factory->user->create();
$post_id = $this->factory->post->create( array( 'post_author' => $user_id ) );

Also, if you need more than just the ID of the object you are
creating, you don’t need to do this:

$post_id = $this->factory->post->create();
$post = get_post( $post_id );

Instead, use the create_and_get() method:

// $post will be an instance of WP_Post 
$post = $this->factory->post->create_and_get();

In this example, we used the post factory, but the same is true
for all of the factories.

I think I’ll mention this to the WordPress docs team. Maybe we can get this stuff into the plugin and theme handbooks.

Update (June 20, 2015): You can also create your own custom factories!

Update (September 27, 2016): In WordPress 4.4 the tests were updated to provide a static factory() method for accessing the factories, although the factory property is still provided via a magic getter.