Fix and re-enable AbstractModelTrackerTests

There used to be a [bug in Django](https://code.djangoproject.com/ticket/30427)
that caused these tests to fail, according to the discussion of #370.
That bug was fixed in Django 4.0.

However, because the test model was using a `FieldTracker`
rather than a `ModelTracker`, all model-specific test cases
in `ModelTrackerTests` continued to fail.

tests/models.py

@@ -249,7 +249,7 @@ class TrackedAbstract(AbstractTracked):
     number = models.IntegerField()
     mutable = MutableField(default=None)
 
-    tracker = FieldTracker()
+    tracker = ModelTracker()
 
 
 class TrackedNotDefault(models.Model):

tests/test_fields/test_field_tracker.py

@@ -1,7 +1,5 @@
 from __future__ import annotations
 
-from unittest import skip
-
 from django.core.cache import cache
 from django.core.exceptions import FieldError
 from django.db import models
@@ -877,7 +875,6 @@ def test_child_fields_not_tracked(self):
         self.assertTrue(self.tracker.has_changed('name2'))
 
 
-@skip("has known failures")
 class AbstractModelTrackerTests(ModelTrackerTests):
 
     tracked_class = TrackedAbstract