core: add a facility to execute functions on the main io_service

Change-Id: I38e5f4ad5ed6798a14e0075fb7a14b792f8b2413
refs: #4683
diff --git a/tests/rib-io-fixture.hpp b/tests/rib-io-fixture.hpp
index e89ba44..ec147d5 100644
--- a/tests/rib-io-fixture.hpp
+++ b/tests/rib-io-fixture.hpp
@@ -56,6 +56,10 @@
   poll();
 
 protected:
+  /** \brief pointer to global main io_service
+   */
+  boost::asio::io_service* g_mainIo = nullptr;
+
   /** \brief pointer to global RIB io_service
    */
   boost::asio::io_service* g_ribIo = nullptr;