Unit tests for some Intent objects and compilers

Unit tests for HostToHost and MultiPointToSinglePoint intents
and intent compilers.

Made Intent classes final to make them immutable.
diff --git a/core/api/src/main/java/org/onlab/onos/net/intent/HostToHostIntent.java b/core/api/src/main/java/org/onlab/onos/net/intent/HostToHostIntent.java
index f420fc2..7a894be6 100644
--- a/core/api/src/main/java/org/onlab/onos/net/intent/HostToHostIntent.java
+++ b/core/api/src/main/java/org/onlab/onos/net/intent/HostToHostIntent.java
@@ -12,7 +12,7 @@
 /**
  * Abstraction of end-station to end-station bidirectional connectivity.
  */
-public class HostToHostIntent extends ConnectivityIntent {
+public final class HostToHostIntent extends ConnectivityIntent {
 
     private final HostId one;
     private final HostId two;