build: move `config.hpp` private header to `detail` subdir

Change-Id: I9f3bb2e288450f002ecec77b52411647ed4fea5a
diff --git a/tests/wscript b/tests/wscript
index 42d537d..b983992 100644
--- a/tests/wscript
+++ b/tests/wscript
@@ -8,4 +8,5 @@
         name='unit-tests',
         source=bld.path.ant_glob('**/*.cpp'),
         use='BOOST_TESTS libndn-nac',
+        includes=top,
         install_path=None)