Exposed MHD_VERSION to test/server.cpp

It turns out that the C preprocessor doesn't complain about undefined
macros used in the arithmetic expressions in `#if` directives (treating
those conditions simply as not satisfied). As a result, PR #1293 had
a slightly different effect from that which was intended. Now taking
the opportunity to correct that subtle mistake.

test/server.cpp

@@ -9,6 +9,9 @@
 #include "../src/tools/stringTools.h"
 
 #include "testing_tools.h"
+
+#include "../src/server/microhttpd_wrapper.h" // for MHD_VERSION
+
 using namespace kiwix::testing;
 
 const std::string ROOT_PREFIX("/ROOT%23%3F");