While we're at it, add tests for EncodedLen and DecodedLen.

Fixes #14803.

Change-Id: I200c72cf11c51669b8d9f70c6e57ece359f7ae61
Reviewed-on: https://go-review.googlesource.com/20649Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>