[1.7.x] Split ignores_nulls_in_unique_constraints feature.

Oracle and SQL Server don't have exactly the same limitations. It's
worth treating them differently.

Backport of cff59bed from master

tests/model_forms/tests.py

@@ -660,7 +660,7 @@ class UniqueTest(TestCase):
         form = TripleForm({'left': '1', 'middle': '3', 'right': '1'})
         self.assertTrue(form.is_valid())
 
-    @skipUnlessDBFeature('ignores_nulls_in_unique_constraints')
+    @skipUnlessDBFeature('supports_nullable_unique_constraints')
     def test_unique_null(self):
         title = 'I May Be Wrong But I Doubt It'
         form = BookForm({'title': title, 'author': self.writer.pk})