crypto/bls: fix bls signature batch unit test (axieinfinity#306)

We randomly observe this failure when running unit test

go test -test.v -run=^TestSignatureBatch_AggregateBatch/common_and_uncommon_messages_in_batch_with_multiple_messages
=== RUN   TestSignatureBatch_AggregateBatch
=== RUN   TestSignatureBatch_AggregateBatch/common_and_uncommon_messages_in_batch_with_multiple_messages
    signature_batch_test.go:643: AggregateBatch() Descriptions got = [test signature bls aggregated signature test signature bls aggregated signature test signature bls aggregated signature], want [bls aggregated signature test signature bls aggregated signature test signature bls aggregated signature test signature]
--- FAIL: TestSignatureBatch_AggregateBatch (0.02s)
    --- FAIL: TestSignatureBatch_AggregateBatch/common_and_uncommon_messages_in_batch_with_multiple_messages (0.02s)

The problem is that the signature sort forgets to swap the description when a
swap occurs. This commit adds the description swap when swap occurs.

crypto/bls/signature_batch_test.go

@@ -3,12 +3,13 @@ package bls
 import (
 	"bytes"
 	"fmt"
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
 	"reflect"
 	"sort"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
 	"github.com/ethereum/go-ethereum/crypto/bls/common"
 )
 
@@ -737,6 +738,7 @@ func (s sorter) Swap(i, j int) {
 	s.set.Signatures[i], s.set.Signatures[j] = s.set.Signatures[j], s.set.Signatures[i]
 	s.set.PublicKeys[i], s.set.PublicKeys[j] = s.set.PublicKeys[j], s.set.PublicKeys[i]
 	s.set.Messages[i], s.set.Messages[j] = s.set.Messages[j], s.set.Messages[i]
+	s.set.Descriptions[i], s.set.Descriptions[j] = s.set.Descriptions[j], s.set.Descriptions[i]
 }
 
 func (s sorter) Less(i, j int) bool {