[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

4 files changed, 4 insertions, 3 deletions

diff --git a/tests/distinct_on_fields/tests.py b/tests/distinct_on_fields/tests.py
index 33ddf80ff4..3e6e6c2358 100644
--- a/tests/distinct_on_fields/tests.py
+++ b/tests/distinct_on_fields/tests.py
@@ -8,6 +8,7 @@ from .models import Tag, Celebrity, Fan, Staff, StaffTag
 
 
 @skipUnlessDBFeature('can_distinct_on_fields')
+@skipUnlessDBFeature('supports_nullable_unique_constraints')
 class DistinctOnTests(TestCase):
     def setUp(self):
         t1 = Tag.objects.create(name='t1')
diff --git a/tests/model_forms/tests.py b/tests/model_forms/tests.py
index 38d1260de0..9a88ff60f0 100644
--- a/tests/model_forms/tests.py
+++ b/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})
diff --git a/tests/model_formsets/tests.py b/tests/model_formsets/tests.py
index c1e01cd6be..85ef430b93 100644
--- a/tests/model_formsets/tests.py
+++ b/tests/model_formsets/tests.py
@@ -661,7 +661,7 @@ class ModelFormsetTest(TestCase):
         self.assertEqual(book1.title, 'Flowers of Evil')
         self.assertEqual(book1.notes, 'English translation of Les Fleurs du Mal')
 
-    @skipUnlessDBFeature('ignores_nulls_in_unique_constraints')
+    @skipUnlessDBFeature('supports_partially_nullable_unique_constraints')
     def test_inline_formsets_with_nullable_unique_together(self):
         # Test inline formsets where the inline-edited object has a
         # unique_together constraint with a nullable member
diff --git a/tests/one_to_one_regress/tests.py b/tests/one_to_one_regress/tests.py
index 17528de311..f14d1eeabd 100644
--- a/tests/one_to_one_regress/tests.py
+++ b/tests/one_to_one_regress/tests.py
@@ -229,7 +229,7 @@ class OneToOneRegressionTests(TestCase):
 
         # Several instances of the origin are only possible if database allows
         # inserting multiple NULL rows for a unique constraint
-        if connection.features.ignores_nulls_in_unique_constraints:
+        if connection.features.supports_nullable_unique_constraints:
             UndergroundBar.objects.create()
 
             # When there are several instances of the origin