native: fix getvalidators to match C# implementation

ValidatorsCount is not initialized at block 0 with C# node (the first voter
initializes it) and until that initialization happens the standby validators
list is being returned as is without sorting.

Fixes state mismatch for the key ffffffff0e00000000000000000000000000000001 in
the first blocks.

It also affects tests as now the first validator is different and it receives
the network fees.

pkg/consensus/consensus_test.go

@@ -213,7 +213,7 @@ func newTestService(t *testing.T) *service {
 }
 
 func getTestValidator(i int) (*privateKey, *publicKey) {
-	key := testchain.PrivateKey(i)
+	key := testchain.PrivateKeyByID(i)
 	return &privateKey{PrivateKey: key}, &publicKey{PublicKey: key.PublicKey()}
 }
 
@@ -241,9 +241,8 @@ func signTx(t *testing.T, feePerByte util.Fixed8, txs ...*transaction.Transactio
 	validators := make([]*keys.PublicKey, 4)
 	privNetKeys := make([]*keys.PrivateKey, 4)
 	for i := 0; i < 4; i++ {
-		privateKey, publicKey := getTestValidator(i)
-		validators[i] = publicKey.PublicKey
-		privNetKeys[i] = privateKey.PrivateKey
+		privNetKeys[i] = testchain.PrivateKey(i)
+		validators[i] = privNetKeys[i].PublicKey()
 	}
 	rawScript, err := smartcontract.CreateMultiSigRedeemScript(3, validators)
 	require.NoError(t, err)