Test ImageShow.show on CIs

[radarhere]

This PR adds a test that actually runs the ImageShow.show method, opening the image. However, to be neat, it only runs on the CIs, where the shown images won't bother users.

[hugovk]

Can we also run it on Azure Pipelines?

CI	env vars
AppVeyor	CI, APPVEYOR
Azure Pipelines	TF_BUILD
Travis CI	CI, CONTINUOUS_INTEGRATION, TRAVIS

There's no similar env var for GitHub Actions yet, it's been requested here:

• https://github.bokerqi.topmunity/t5/GitHub-Actions/Have-the-CI-environment-variable-set-by-default/m-p/32125/highlight/true#M1013

Perhaps add an on_ci() that checks for CI and TF_BUILD? Then if GHA adds CI it'll be covered.

[radarhere]

Okay, done.

[nulano]

Note, that on Windows this will always either succeed (AppVeyor), or hang (GitHub Actions). It can't fail, even if there is no application associated with the file format.