Fixed #29658 -- Registered model lookups in tests with a context manager.

tests/db_functions/math/test_abs.py

@@ -3,6 +3,7 @@ from decimal import Decimal
 from django.db.models import DecimalField
 from django.db.models.functions import Abs
 from django.test import TestCase
+from django.test.utils import register_lookup
 
 from ..models import DecimalModel, FloatModel, IntegerModel
 
@@ -40,11 +41,8 @@ class AbsTests(TestCase):
         self.assertEqual(obj.big, -obj.big_abs)
 
     def test_transform(self):
-        try:
-            DecimalField.register_lookup(Abs)
+        with register_lookup(DecimalField, Abs):
             DecimalModel.objects.create(n1=Decimal('-1.5'), n2=Decimal('0'))
             DecimalModel.objects.create(n1=Decimal('-0.5'), n2=Decimal('0'))
             objs = DecimalModel.objects.filter(n1__abs__gt=1)
             self.assertQuerysetEqual(objs, [Decimal('-1.5')], lambda a: a.n1)
-        finally:
-            DecimalField._unregister_lookup(Abs)