[1.8.x] Moved non-documented auth test models to the new test location.

Backport of 5ab327a389 from master

tests/auth_tests/models/invalid_models.py

@@ -0,0 +1,36 @@
+from django.contrib.auth.models import AbstractBaseUser
+from django.db import models
+
+
+class CustomUserNonUniqueUsername(AbstractBaseUser):
+    "A user with a non-unique username"
+    username = models.CharField(max_length=30)
+
+    USERNAME_FIELD = 'username'
+
+    class Meta:
+        app_label = 'auth'
+
+
+class CustomUserNonListRequiredFields(AbstractBaseUser):
+    "A user with a non-list REQUIRED_FIELDS"
+    username = models.CharField(max_length=30, unique=True)
+    date_of_birth = models.DateField()
+
+    USERNAME_FIELD = 'username'
+    REQUIRED_FIELDS = 'date_of_birth'
+
+    class Meta:
+        app_label = 'auth'
+
+
+class CustomUserBadRequiredFields(AbstractBaseUser):
+    "A user with a USERNAME_FIELD that appears in REQUIRED_FIELDS (invalid)"
+    username = models.CharField(max_length=30, unique=True)
+    date_of_birth = models.DateField()
+
+    USERNAME_FIELD = 'username'
+    REQUIRED_FIELDS = ['username', 'date_of_birth']
+
+    class Meta:
+        app_label = 'auth'