I seem to have got past most of the addon test mode issues on MacOS by using homebrew's nginx, and ssl using a keychain generated self signed key that is customized to include the subjectaltnam and authorized for ssl in the keychain. There was some tweaking of the nginx.conf file to contain the proper CORS http headers. If you don't have your self signed cert completely authenticated in Keychain, you won't even see responses sent back from the localhost web server when requested by C3. Also, port's nginx doesn't even install, so avoid that.
It sounds more painful than it was.
Is there any chance you could share some more detailed tips on how to do this? I've tried with no luck in Windows with a python server I found, also with XAMPP... in both cases using a self-signed certificate. Chrome keeps not-accepting the certificate so I haven't been able to load addons.
I can try on MacOS if it worked for you, so any details about your configuration would be very much appreciated