Use -id as a secondary key when retrieving latest page revision.

This fixes various unit tests on MySQL, where multiple revisions are committed in the space of one second, and the resolution of the created_at timestamp is insufficient to distinguish them.

wagtail/wagtailcore/models.py

@@ -481,7 +481,7 @@ class Page(six.with_metaclass(PageBase, MP_Node, ClusterableModel, index.Indexed
         return revision
 
     def get_latest_revision(self):
-        return self.revisions.order_by('-created_at').first()
+        return self.revisions.order_by('-created_at', '-id').first()
 
     def get_latest_revision_as_page(self):
         latest_revision = self.get_latest_revision()
@@ -1097,7 +1097,7 @@ class PageRevision(models.Model):
             # special case: a revision without an ID is presumed to be newly-created and is thus
             # newer than any revision that might exist in the database
             return True
-        latest_revision = PageRevision.objects.filter(page_id=self.page_id).order_by('-created_at').first()
+        latest_revision = PageRevision.objects.filter(page_id=self.page_id).order_by('-created_at', '-id').first()
         return (latest_revision == self)
 
     def publish(self):