Add benchmark for catch_test_macros.hpp inclusion

benchmarks/only_include.cpp

@@ -0,0 +1,16 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
+/**\file 
+ * "empty" include of catch_test_macros.hpp
+ * 
+ * This file does only one thing: include `catch2/catch_test_macros.hpp`,
+ * so that we can measure the compilation cost of the header.
+ */
+
+#include <catch2/catch_test_macros.hpp>