tests: introduce unit testing

As a sample, this commit also includes a test suite for
ndn::ping::client::StatisticsCollector class.

refs #2795

Change-Id: Ia0c7522ae26bee1cc9d172c7379ab62f84325ed7
diff --git a/tools/ping/client/statistics-collector.hpp b/tools/ping/client/statistics-collector.hpp
index 685ff1f..76278db 100644
--- a/tools/ping/client/statistics-collector.hpp
+++ b/tools/ping/client/statistics-collector.hpp
@@ -64,6 +64,13 @@
   StatisticsCollector(Ping& ping, const Options& options);
 
   /**
+   * Returns ping statistics as structure
+   */
+  Statistics
+  computeStatistics();
+
+PUBLIC_WITH_TESTS_ELSE_PRIVATE:
+  /**
    * Called on ping response received
    * @param rtt round trip time
    */
@@ -76,12 +83,6 @@
   void
   recordTimeout();
 
-  /**
-   * Returns ping statistics as structure
-   */
-  Statistics
-  computeStatistics();
-
 private:
   Ping& m_ping;
   const Options& m_options;