Split test cases

Author: tudorvaran

tests/test_isnull.py

@@ -33,10 +33,19 @@ def test_isnull_lookup_valid_values(self):
         assert self.TestModel.objects.filter(text__isnull=True).exists()
         assert self.TestModel.objects.filter(text__isnull=False).exists()
 
-    def test_isnull_lookup_invalid_values(self):
-        """Test whether isnull crashes with invalid values."""
+    def test_isnull_lookup_null(self):
+        """Test whether isnull crashes with None as value."""
+
         with pytest.raises(ValueError):
             assert self.TestModel.objects.filter(text__isnull=None).exists()
 
-        with pytest.raises(ValueError):
+    def test_isnull_lookup_string(self):
+        """Test whether isnull properly works with string values on the
+        corresponding Django version."""
+        if django.VERSION < (4, 0):
             assert self.TestModel.objects.filter(text__isnull="True").exists()
+        else:
+            with pytest.raises(ValueError):
+                assert self.TestModel.objects.filter(
+                    text__isnull="True"
+                ).exists()