Test depends-on/depends-on-action action

[gforcada]

This is an idea coming from @petschki to be able to test a PR that depends on (multiple) PRs.

https://github.com/depends-on/depends-on-action

[davisagli]

Looks interesting. I like that it uses something declared in the pull request description, rather than in a file in the repository (which could accidentally get merged)