Added missing on_delete to SearchTestSubObject.parent

CONTRIBUTORS.rst

@@ -254,6 +254,7 @@ Contributors
 * jcronyn
 * Ben Sturmfels
 * Anselm Bradford
+* Mads Jensen
 
 Translators
 ===========

wagtail/tests/search/models.py

@@ -85,7 +85,7 @@ class AnotherSearchTestChild(SearchTest):
 
 
 class SearchTestSubObject(models.Model):
-    parent = models.ForeignKey(SearchTest, related_name='subobjects')
+    parent = models.ForeignKey(SearchTest, related_name='subobjects', on_delete=models.CASCADE)
     name = models.CharField(max_length=255)
 
     def __str__(self):