[#277] netmap: Allow more uint64 config values

NEO VM uses little-endian format for integers,
however the resulting byte slice can contain less than 8 bytes.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

netmap/network_info_decode_test.go

@@ -0,0 +1,28 @@
+package netmap
+
+import (
+	"math/big"
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/encoding/bigint"
+	"github.com/stretchr/testify/require"
+)
+
+func TestDecodeUint64(t *testing.T) {
+	testCases := []uint64{
+		0,
+		12,
+		129,
+		0x1234,
+		0x12345678,
+		0x1234567891011,
+	}
+
+	for _, expected := range testCases {
+		val := bigint.ToBytes(big.NewInt(int64(expected)))
+
+		actual, err := decodeConfigValueUint64(val)
+		require.NoError(t, err)
+		require.Equal(t, expected, actual)
+	}
+}