Add guidance how to write tests

[hhorak]

I've tried to write-up what I'd like to say every image maintainers when he asks about how to create tests for container images. I'm not sure whether this is the form that is expected for this guide, but we can work on it.

[bexelbie]

I added mostly language edits.

[hhorak]

Sorry for shameful delay, finally got to fix the PR and include suggested edits.

[eliskasl]

@hhorak, can you please link the new chapter from the index.adoc file so it shows up in the book? thanks

[hhorak]

@eliskasl Done.