Transfer completed Go output without copies

internal/gotool/performance_test.go

@@ -28,7 +28,7 @@ func BenchmarkPerformanceCombinedOutput(b *testing.B) {
 				if written, err := writer.Write(input); err != nil || written != len(input) {
 					b.Fatalf("write captured output = %d, err %v; want %d bytes", written, err, len(input))
 				}
-				captured := writer.Bytes()
+				captured := writer.take()
 				if len(captured) != len(input) {
 					b.Fatalf("captured output = %d bytes, want %d", len(captured), len(input))
 				}