cmd/swarm, swarm/api/http, swarm/bmt, swarm/fuse, swarm/network/stream, swarm/storage, swarm/storage/encryption, swarm/testutil: use pseudo-random instead of crypto-random for test files content generation (#18083)

- Replace "crypto/rand" to "math/rand" for files content generation
- Remove swarm/network_test.go.Shuffle and swarm/btm/btm_test.go.Shuffle - because go1.9 support dropped (see https://github.com/ethereum/go-ethereum/pull/17807 and comments to swarm/network_test.go.Shuffle)

swarm/network/stream/snapshot_sync_test.go

@@ -17,9 +17,7 @@ package stream
 
 import (
 	"context"
-	crand "crypto/rand"
 	"fmt"
-	"io"
 	"os"
 	"runtime"
 	"sync"
@@ -39,6 +37,7 @@ import (
 	"github.com/ethereum/go-ethereum/swarm/state"
 	"github.com/ethereum/go-ethereum/swarm/storage"
 	mockdb "github.com/ethereum/go-ethereum/swarm/storage/mock/db"
+	"github.com/ethereum/go-ethereum/swarm/testutil"
 )
 
 const MaxTimeout = 600
@@ -603,7 +602,7 @@ func uploadFileToSingleNodeStore(id enode.ID, chunkCount int, lstore *storage.Lo
 	size := chunkSize
 	var rootAddrs []storage.Address
 	for i := 0; i < chunkCount; i++ {
-		rk, wait, err := fileStore.Store(context.TODO(), io.LimitReader(crand.Reader, int64(size)), int64(size), false)
+		rk, wait, err := fileStore.Store(context.TODO(), testutil.RandomReader(i, size), int64(size), false)
 		if err != nil {
 			return nil, err
 		}