[2507] Suppress another false positive

first is used in the condition during the next iteration of the loop.

src/bin/dhcp6/tests/config_parser_unittest.cc

@@ -99,6 +99,7 @@ public:
             if (!first) {
                 stream << ", ";
             } else {
+                // cppcheck-suppress unreadVariable
                 first = false;
             }
             if (param.first == "name") {