Test more Scala CLI snippets for output (printed results)

[MateuszKubuszok]

Currently most Scala CLI snippets in the docs are tested (with ScalaCLI.md Spec) either if they merely compile or for failures (// expected error:.

Snippets testing for successful result could println some result and have it checked with // expected output:.

This would be relatively simple to do:

1. run sbt publish-local-for-tests to publish artifacts locally
2. then scala-cli run scripts/test-snippets.scala -- --extra "chimney-version=[version from the artifacts]" --test-only '*' "$PWD/docs/docs" - it should run all tests
3. then open some .md file, look fora a snippet without output/error test
4. add

   // expected output:
   // make it fail

5. rerun scala-cli run scripts/test-snippets.scala -- ... to verify that it fails
6. modify example to make it pass
7. go to 3. and repeat