cmd/dist: copy trailing text more directly in testJSONFilter.process

Use io.Copy¹ that matches the comment more closely, avoids the
possibility of needing a bigger array, and is slightly shorter.
Its downside is that it takes two w.Write calls instead of one.

¹ Admittedly, it was temping to use io.CopyBuffer since the 'data'
  byte slice becomes a viable buffer after its contents are written.
  I resisted that temptation for two reasons.

  One, it would need the io.Reader returned by dec.Buffered() (currently
  a *bytes.Reader) to not implement the io.WriterTo interface for any
  chance of making a positive difference. This seems not very likely.

  Two, to avoid burdening anyone with determining that io.CopyBuffer
  won't panic without 'if len(data) == 0 && data != nil { data = nil }'
  because json.Marshal never returns an empty but non-nil byte slice.

Change-Id: I33c53d9d990f6ee79cd3ab90f12e3b575b9ebe72
Reviewed-on: https://go-review.googlesource.com/c/go/+/497736
Reviewed-by: Austin Clements <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
TryBot-Bypass: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>

Author: dmitshur

src/cmd/dist/testjson.go

@@ -100,11 +100,10 @@ func (f *testJSONFilter) process(line []byte) {
 					// Should never happen.
 					panic(fmt.Sprintf("failed to round-trip JSON %q: %s", string(line), err))
 				}
+				f.w.Write(data)
 				// Copy any trailing text. We expect at most a "\n" here, but
 				// there could be other text and we want to feed that through.
-				extra, _ := io.ReadAll(dec.Buffered())
-				data = append(data, extra...)
-				f.w.Write(data)
+				io.Copy(f.w, dec.Buffered())
 				return
 			}
 		}