[1570] a supplemental check to the in-memory data source about DS lookup:

confirm DS query at a grandparent results in normal delegation.
(the code was already correct, just checking it explicitly)

src/lib/datasrc/tests/memory_datasrc_unittest.cc

@@ -697,11 +697,13 @@ TEST_F(InMemoryZoneFinderTest, delegationWithDS) {
     EXPECT_EQ(SUCCESS, zone_finder_.add(rr_child_ds_));
 
     // Normal types of query should result in delegation, but DS query
-    // should be considered in-zone.
+    // should be considered in-zone (but only exactly at the delegation point).
     findTest(Name("child.example.org"), RRType::A(), ZoneFinder::DELEGATION,
              true, rr_child_ns_);
     findTest(Name("child.example.org"), RRType::DS(), ZoneFinder::SUCCESS,
              true, rr_child_ds_);
+    findTest(Name("grand.child.example.org"), RRType::DS(),
+             ZoneFinder::DELEGATION, true, rr_child_ns_);
 
     // There's nothing special for DS query at the zone apex.  It would
     // normally result in NXRRSET.