wallet_rpc_server: longer timeout for stop_mining

That RPC will wait for mining to actually stop, which can be a while
if randomx has just started on randomx_init_dataset.

This fixes occasional failures in the mining functional test

src/wallet/wallet_rpc_server.cpp

@@ -3254,7 +3254,7 @@ namespace tools
     if (!m_wallet) return not_open(er);
     cryptonote::COMMAND_RPC_STOP_MINING::request daemon_req;
     cryptonote::COMMAND_RPC_STOP_MINING::response daemon_res;
-    bool r = m_wallet->invoke_http_json("/stop_mining", daemon_req, daemon_res);
+    bool r = m_wallet->invoke_http_json("/stop_mining", daemon_req, daemon_res, std::chrono::seconds(60)); // this waits till stopped, and if randomx has just started initializing its dataset, it might be a while
     if (!r || daemon_res.status != CORE_RPC_STATUS_OK)
     {
       er.code = WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR;