Fix test-program failures

Due to recent changes, mxs::MessageQueue::init() must be called
explicitly, if a monitor is created.

server/core/test/test_modulecmd.cc

@@ -17,6 +17,7 @@
 
 #include <maxscale/alloc.h>
 #include <maxscale/dcb.h>
+#include <maxscale/messagequeue.hh>
 #include <maxscale/paths.h>
 #include <maxscale/modulecmd.h>
 #include <maxscale/session.h>
@@ -464,6 +465,8 @@ int main(int argc, char **argv)
 {
     int rc = 0;
 
+    mxs::MessageQueue::init();
+
     rc += test_arguments();
     rc += test_optional_arguments();
     rc += test_module_errors();