You can use this Node JS module: https://www.npmjs.com/package/local-ssl-proxy
I have used this module for another case and works great. Just download Node JS, Run the Console and enter these commands:
npm install -g local-ssl-proxy local-ssl-proxy --source 9090 --target 8080
after that I had to let my browser accept selfsigning certificates for https://localhost:9090 by this howto: https://conetrix.com/blog/how-to-trust-a-self-signed-certificate-in-ie-9
Maybe this is also a solution for you..
I am not sure this will test your secure local connection from your local app, but you can try it.
When implementing SAML I set up a production like IdP tenant with multiple attributes/assertions using https://stubidp.sustainsys.com/c7e1fc45-5f25-4a5e-b27b-8acb2563312e/ . Here you can specify the " Assertion Consumer Service Url ", you may can try a secure URL.
Based on these results I proceed to the app environment and set it with the production IdP configuration. On this way you can test custom login logic etc.