It would be nice if there was a way to tell compilers not to drop the lock stack and thus issue warnings when accessing a symbol that needs a lock inside `Assume`/`Assert`.

Hit by (at least): @ajtowns , @jnewbery , me.
{
    CHash256 out_sha;
    FILE* out_file = nullptr;
    if (const char* out_path = getenv("ARGS_MERGE_TEST_OUT")) {
        out_file = fsbridge::fopen(out_path, "w");
        if (!out_file) throw std::system_error(errno, std::generic_category(), "fopen failed");
    }
    ForEachMergeSetup([&](const ActionList& arg_actions, const ActionList& conf_actions, bool soft_set, bool force_set,
                          const std::string& section, const std::string& network, bool net_specific) {
        TestArgsManager parser;
        LOCK(parser.cs_args);
        std::string desc = "net=";