nixos/geth: fix failing test

Run authrpc on different ports for main and testnet to avoid collision.

nixos/tests/geth.nix

···
         enable = true;
         port = 18545;
       };
+      authrpc = {
+        enable = true;
+        port = 18551;
+      };
     };
   };
 
···
     machine.wait_for_open_port(18545)
 
     machine.succeed(
-        'geth attach --exec eth.blockNumber http://localhost:8545 | grep \'^0$\' '
+        'geth attach --exec "eth.blockNumber" http://localhost:8545 | grep \'^0$\' '
     )
 
     machine.succeed(
-        'geth attach --exec "eth.chainId()" http://localhost:18545 | grep \'"0x5"\' '
+        'geth attach --exec "eth.blockNumber" http://localhost:18545 | grep \'^0$\' '
     )
   '';
 })