pic: add a test
pic: add a test

It's hard to write a normal test because ShowImage writes
to os.Stdout unconditionally. Add an example test instead.

Change-Id: I52e02908e94aa212a05c058edb4f504851b5bb8d
Reviewed-on: https://go-review.googlesource.com/c/tour/+/232866
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>

golang/tour,zorion/go-tour-ca,zorion/go-tour-ca