nixos/tests/wasabibackend: fix bitcoind config

Since c6017d98 services.bitcoind is submodule.

nixos/tests/wasabibackend.nix

···
           port = 18332;
         };
       };
-      services.bitcoind = {
+      services.bitcoind."testnet" = {
         enable = true;
         testnet = true;
         rpc.users = {