Explicitly test for canonicity

This will come in handy later, with more complex test cases.

src/include/interval-map/interval-map.hh

@@ -2,6 +2,9 @@
 
 #include <map>
 
+// Testing class forward declaration
+class IntervalMapTest;
+
 namespace amby {
 
 template <typename K, typename V> class interval_map {
@@ -21,6 +24,9 @@ public:
         return std::prev(it)->second;
     }
 
+    // Used in testing
+    friend class ::IntervalMapTest;
+
 private:
     V init_;
     std::map<K, V> underlying_{};