tools: set CanBePrefix in autoconfig

Also, rewrite RoutablePrefixesDataset test suite to use
SegmentFetcher.

refs #4581

Change-Id: I68e0ffcd59dcbae0856bfa2977545f06482f3515
diff --git a/tools/ndn-autoconfig/multicast-discovery.cpp b/tools/ndn-autoconfig/multicast-discovery.cpp
index 1baf831..267dfd9 100644
--- a/tools/ndn-autoconfig/multicast-discovery.cpp
+++ b/tools/ndn-autoconfig/multicast-discovery.cpp
@@ -130,8 +130,9 @@
 MulticastDiscovery::requestHubData()
 {
   Interest interest(HUB_DISCOVERY_PREFIX);
-  interest.setInterestLifetime(HUB_DISCOVERY_INTEREST_LIFETIME);
+  interest.setCanBePrefix(true);
   interest.setMustBeFresh(true);
+  interest.setInterestLifetime(HUB_DISCOVERY_INTEREST_LIFETIME);
 
   m_face.expressInterest(interest,
     [this] (const Interest&, const Data& data) {