The code was fixed in CL 108559 but the testing TODO was not implemented.

Updates #22936

Change-Id: I20a703260a181bbcf5f87609d6fb8221a182be1a
Reviewed-on: https://go-review.googlesource.com/125038
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>