Fixed #31395 -- Made setUpTestData enforce in-memory data isolation.

Since it's introduction in Django 1.8 setUpTestData has been suffering
from a documented but confusing caveat due to its sharing of attributes
assigned during its execution with all test instances.

By keeping track of class attributes assigned during the setUpTestData
phase its possible to ensure only deep copies are provided to test
instances on attribute retreival and prevent manual setUp gymnastic to
work around the previous lack of in-memory data isolation.

Thanks Adam Johnson for the extensive review.

tests/test_utils/models.py

@@ -12,4 +12,4 @@ class Person(models.Model):
 
 class PossessedCar(models.Model):
     car = models.ForeignKey(Car, models.CASCADE)
-    belongs_to = models.ForeignKey(Person, models.CASCADE)
+    belongs_to = models.ForeignKey(Person, models.CASCADE, related_name='possessed_cars')