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/storage/common_test.go

@@ -19,7 +19,6 @@ package storage
 import (
 	"bytes"
 	"context"
-	"crypto/rand"
 	"flag"
 	"fmt"
 	"io"
@@ -31,7 +30,7 @@ import (
 
 	"github.com/ethereum/go-ethereum/log"
 	ch "github.com/ethereum/go-ethereum/swarm/chunk"
-	colorable "github.com/mattn/go-colorable"
+	"github.com/mattn/go-colorable"
 )
 
 var (
@@ -151,10 +150,6 @@ func mget(store ChunkStore, hs []Address, f func(h Address, chunk Chunk) error)
 	return err
 }
 
-func testDataReader(l int) (r io.Reader) {
-	return io.LimitReader(rand.Reader, int64(l))
-}
-
 func (r *brokenLimitedReader) Read(buf []byte) (int, error) {
 	if r.off+len(buf) > r.errAt {
 		return 0, fmt.Errorf("Broken reader")