Per #11257 all examples should be in external test files.

Additionally, doing so makes this example playable.

Updates #24352. (Albeit tangentially).

Change-Id: I77ab4655107f61db2e9d21a608b73ace3a230fb2
Reviewed-on: https://go-review.googlesource.com/101285Reviewed-by: Robert Griesemer <gri@golang.org>