tests: Use realistic custom user model

The previous test setup inherited from AbstractUser thus MyCustomUser
still had a username field.

The problems people are having when using the admin_client fixture in
combination with a custom user model are due to the username field
not being present.

This change accounts for the more realistic scenario.

tests/test_fixtures.py

@@ -483,15 +483,40 @@ def test_with_live_server(live_server):
 def test_custom_user_model(django_testdir, username_field):
     django_testdir.create_app_file(
         """
-        from django.contrib.auth.models import AbstractUser
+        from django.contrib.auth.models import AbstractBaseUser, BaseUserManager, PermissionsMixin
         from django.db import models
 
-        class MyCustomUser(AbstractUser):
+        class MyCustomUserManager(BaseUserManager):
+            def create_user(self, {username_field}, password=None, **extra_fields):
+                extra_fields.setdefault('is_staff', False)
+                extra_fields.setdefault('is_superuser', False)
+                user = self.model({username_field}={username_field}, **extra_fields)
+                user.set_password(password)
+                user.save()
+                return user
+
+            def create_superuser(self, {username_field}, password=None, **extra_fields):
+                extra_fields.setdefault('is_staff', True)
+                extra_fields.setdefault('is_superuser', True)
+                return self.create_user(
+                    {username_field}={username_field},
+                    password=password,
+                    **extra_fields
+                )
+
+        class MyCustomUser(AbstractBaseUser, PermissionsMixin):
+            email = models.EmailField(max_length=100, unique=True)
             identifier = models.CharField(unique=True, max_length=100)
+            is_staff = models.BooleanField(
+                'staff status',
+                default=False,
+                help_text='Designates whether the user can log into this admin site.'
+            )
 
-            USERNAME_FIELD = '%s'
-        """
-        % (username_field),
+            objects = MyCustomUserManager()
+
+            USERNAME_FIELD = '{username_field}'
+        """.format(username_field=username_field),
         "models.py",
     )
     django_testdir.create_app_file(
@@ -551,19 +576,13 @@ class Migration(migrations.Migration):
                 ('password', models.CharField(max_length=128, verbose_name='password')),
                 ('last_login', models.DateTimeField(null=True, verbose_name='last login', blank=True)),
                 ('is_superuser', models.BooleanField(default=False, help_text='Designates that this user has all permissions without explicitly assigning them.', verbose_name='superuser status')),
-                ('username', models.CharField(error_messages={'unique': 'A user with that username already exists.'}, max_length=30, validators=[django.core.validators.RegexValidator(r'^[\\w.@+-]+$', 'Enter a valid username. This value may contain only letters, numbers and @/./+/-/_ characters.', 'invalid')], help_text='Required. 30 characters or fewer. Letters, digits and @/./+/-/_ only.', unique=True, verbose_name='username')),
-                ('first_name', models.CharField(max_length=30, verbose_name='first name', blank=True)),
-                ('last_name', models.CharField(max_length=30, verbose_name='last name', blank=True)),
-                ('email', models.EmailField(max_length=254, verbose_name='email address', blank=True)),
+                ('email', models.EmailField(error_messages={'unique': 'A user with that email address already exists.'}, max_length=100, unique=True, verbose_name='email address')),
                 ('is_staff', models.BooleanField(default=False, help_text='Designates whether the user can log into this admin site.', verbose_name='staff status')),
-                ('is_active', models.BooleanField(default=True, help_text='Designates whether this user should be treated as active. Unselect this instead of deleting accounts.', verbose_name='active')),
-                ('date_joined', models.DateTimeField(default=django.utils.timezone.now, verbose_name='date joined')),
                 ('identifier', models.CharField(unique=True, max_length=100)),
                 ('groups', models.ManyToManyField(related_query_name='user', related_name='user_set', to='auth.Group', blank=True, help_text='The groups this user belongs to. A user will get all permissions granted to each of their groups.', verbose_name='groups')),
                 ('user_permissions', models.ManyToManyField(related_query_name='user', related_name='user_set', to='auth.Permission', blank=True, help_text='Specific permissions for this user.', verbose_name='user permissions')),
             ],
             options={
-                'abstract': False,
                 'verbose_name': 'user',
                 'verbose_name_plural': 'users',
             },