test: don't use ddebug() before it's defined

I'm pretty sure this is not the only case, but it's the one I recently
noticed. Even though we call ddebug() from a function, that function is
called before ddebug() is defined, resulting in the same issue as if we
called just ddebug() in its place, i.e.:

..//test-functions: line 276: ddebug: command not found

test/test-functions

@@ -273,7 +273,7 @@ is_built_with_asan() {
     local _bin="${1:?}"
 
     if ! type -P objdump >/dev/null; then
-        ddebug "Failed to find objdump. Assuming systemd hasn't been built with ASAN."
+        echo "Failed to find objdump, assuming systemd hasn't been built with ASAN."
         return 1
     fi