lib/random: unify random string generation into random.String

This was factored from fstest as we were including the testing
enviroment into the main binary because of it.

This was causing opening the browser to fail because of 8243ff8bc8.

fs/operations/operations.go

@@ -8,7 +8,6 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
-	"math/rand"
 	"path"
 	"path/filepath"
 	"sort"
@@ -28,6 +27,7 @@ import (
 	"github.com/rclone/rclone/fs/march"
 	"github.com/rclone/rclone/fs/object"
 	"github.com/rclone/rclone/fs/walk"
+	"github.com/rclone/rclone/lib/random"
 	"github.com/rclone/rclone/lib/readers"
 	"golang.org/x/sync/errgroup"
 )
@@ -1666,7 +1666,7 @@ func moveOrCopyFile(ctx context.Context, fdst fs.Fs, fsrc fs.Fs, dstFileName str
 	// to avoid issues with certain remotes and avoid file deletion.
 	if !cp && fdst.Name() == fsrc.Name() && fdst.Features().CaseInsensitive && dstFileName != srcFileName && strings.ToLower(dstFilePath) == strings.ToLower(srcFilePath) {
 		// Create random name to temporarily move file to
-		tmpObjName := dstFileName + "-rclone-move-" + random(8)
+		tmpObjName := dstFileName + "-rclone-move-" + random.String(8)
 		_, err := fdst.NewObject(ctx, tmpObjName)
 		if err != fs.ErrorObjectNotFound {
 			if err == nil {
@@ -1730,17 +1730,6 @@ func moveOrCopyFile(ctx context.Context, fdst fs.Fs, fsrc fs.Fs, dstFileName str
 	return err
 }
 
-// random generates a pseudorandom alphanumeric string
-func random(length int) string {
-	randomOutput := make([]byte, length)
-	possibleCharacters := "123567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
-	rand.Seed(time.Now().Unix())
-	for i := range randomOutput {
-		randomOutput[i] = possibleCharacters[rand.Intn(len(possibleCharacters))]
-	}
-	return string(randomOutput)
-}
-
 // MoveFile moves a single file possibly to a new name
 func MoveFile(ctx context.Context, fdst fs.Fs, fsrc fs.Fs, dstFileName string, srcFileName string) (err error) {
 	return moveOrCopyFile(ctx, fdst, fsrc, dstFileName, srcFileName, false)