You can upload your files to hosted test and test that way instead. Granted to make a change your have to go back to local test and then reupload a new zip, which is what you would do if you were uploading to a “real” website host.
The bare minimum is HTML/JS/CSS.
Knowing something about servers is useful so you can improve your test environemnt and/or provide an EBS.
EBS being a backend API that your extension might need and that will need SSL too, which might be handled by a hosting provider.
You can host your extension for testing on a “real website” just like a real website and then you are just uploading files to a shared host/whatever hosting, just like a real website.
So yes the minimum you need to know is HTML, JS and css. As an extension works just like a “real website” as essentially it is just just a website.
So you can upload your extension to a “regular” website host and use that URL in your testing base URI.
One of my test extensions even uses GitHub pages to run the hosting. So GitHub handles the hosting and SSL for me, for example.
So you have options on how to deplay you extension/website/files for testing with.
You don’t need any server knowledge if you are using a hosting provider.
Edit: and as noted in my blog post, you can use ngrok to SSL termiante and tunnel to your node http-server and then ngrok handles SSL and making your extension “world accessble” if you need to test with other people (prior to hosted testing)