Restore deleted terminal screenshot in code demo for unit testing (#6480

)

## What are you changing in this pull request and why?

Similar to #5632,
restore the other image that was accidentally deleted by
#5615.

Once this is merged, all the images should load for this blog post
again:
https://docs.getdbt.com/blog/announcing-unit-testing

## Checklist
- [x] I have reviewed the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.