I really loves Caddy, it has replaced nginx in my go-to HTTP server. It's really simple and powerfull, one nice example is how simple you can deploy applications with zero downtine [1].

On testing HTTP requests, I'd suggest Hurl [2], (I'm one of the maintainer). You will trade Ruby unit tests for a tailored HTTP text file format but I think it can shine for this kind of migration. Tests will be run in parallel, really fast.

For instance, test_site_is_up.rb will be replaced by a simple text file:

  GET https://alexwlchan.net
  HTTP 200
  [Asserts]
  body contains "This website is a place to share stuff I find interesting or fun."
  
  
  GET https://alexwlchan.net/articles/
  HTTP 200
  [Asserts]
  body contains "Articles"
  
  # etc...

Testing alternate domains (test_alternate_domains.rb) could be:

  GET http://alexwlchan.net
  HTTP 308
  Location: https://alexwlchan.net/
  
  
  GET http://alexwlchan.net/contact/
  HTTP 301
  Location https://alexwlchan.net/
  
  # etc...

Give it a try!

[1]: https://www.lambrospetrou.com/articles/server-deploy-scripts...

[2]: https://hurl.dev