Print the byte unit once in the summary line (closes #17)

formatBytes already ends in a unit, so the summary line read "9 B
bytes" or "1.0 KiB bytes". The format string in Stats.Print no longer
adds the word "bytes".

Print now takes the io.Writer to print to; its callers pass os.Stderr,
and the new test passes a buffer to read the line for one small and
one large byte count.

Model: opus-5-5

attrsum.go

@@ -71,13 +71,13 @@ func (s *Stats) Duration() time.Duration {
 	return time.Since(s.StartTime)
 }
 
-func (s *Stats) Print(opts *options, operation string) {
+func (s *Stats) Print(w io.Writer, opts *options, operation string) {
 	if opts.quiet {
 		return
 	}
 
-	fmt.Fprintf(os.Stderr,
-		"\n%s complete: %d files processed, %d skipped, %d failed, %s bytes in %s\n",
+	_, _ = fmt.Fprintf(w,
+		"\n%s complete: %d files processed, %d skipped, %d failed, %s in %s\n",
 		operation,
 		s.FilesProcessed,
 		s.FilesSkipped,
@@ -227,7 +227,7 @@ func runOverPaths(
 	}
 
 	finishBar(bar)
-	stats.Print(opts, op)
+	stats.Print(os.Stderr, opts, op)
 
 	return nil
 }
@@ -481,7 +481,7 @@ func runCheck(opts *options, args []string, cont bool) error {
 		if perr != nil {
 			if !cont {
 				finishBar(bar)
-				stats.Print(opts, "check")
+				stats.Print(os.Stderr, opts, "check")
 
 				return perr
 			}
@@ -491,7 +491,7 @@ func runCheck(opts *options, args []string, cont bool) error {
 	}
 
 	finishBar(bar)
-	stats.Print(opts, "check")
+	stats.Print(os.Stderr, opts, "check")
 
 	return finalErr
 }