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/testutil/file.go

@@ -17,8 +17,10 @@
 package testutil
 
 import (
+	"bytes"
 	"io"
 	"io/ioutil"
+	"math/rand"
 	"os"
 	"strings"
 	"testing"
@@ -42,3 +44,22 @@ func TempFileWithContent(t *testing.T, content string) string {
 	}
 	return tempFile.Name()
 }
+
+// RandomBytes returns pseudo-random deterministic result
+// because test fails must be reproducible
+func RandomBytes(seed, length int) []byte {
+	b := make([]byte, length)
+	reader := rand.New(rand.NewSource(int64(seed)))
+	for n := 0; n < length; {
+		read, err := reader.Read(b[n:])
+		if err != nil {
+			panic(err)
+		}
+		n += read
+	}
+	return b
+}
+
+func RandomReader(seed, length int) *bytes.Reader {
+	return bytes.NewReader(RandomBytes(seed, length))
+}