Fix use of Boost unit test framework in AttribStringParser_test

GaudiKernel/tests/src/test_AttribStringParser.cpp

 /***********************************************************************************\
-* (c) Copyright 1998-2019 CERN for the benefit of the LHCb and ATLAS collaborations *
+* (c) Copyright 1998-2023 CERN for the benefit of the LHCb and ATLAS collaborations *
 *                                                                                   *
 * This software is distributed under the terms of the Apache version 2 licence,     *
 * copied verbatim in the file "LICENSE".                                            *
@@ -17,7 +17,7 @@
 #include <boost/regex.hpp>
 
 #define BOOST_TEST_MODULE AttribStringParser_test
-#include <boost/test/included/unit_test.hpp>
+#include <boost/test/unit_test.hpp>
 
 BOOST_AUTO_TEST_CASE( test_case ) {
   using Gaudi::Utils::AttribStringParser;