One of the most common ways to handle automatic authentication is by saving a browser cookie that stores some secret login key. Although this is often used for users, it should also work when just testing in development. Here is the basic idea:
1. You log in on your development frontend
2. Your backend generates a random hash as a secret key and saves this key to a database or file and as a browser cookie.
3. When you reload the site, the frontend sends a request to the backend asking to compare secret keys. If the secret key exists in the backend, this means you can auto authenticate. Now, you can simply load the information from your database related to that secret key.

Some optional (but important) notes:
When saving the cookies in the backend, you can set the cookie as secure so that it sends via HTTPS or SSL. You can also set the cookie as HttpOnly, which hides the cookie from the frontend so no bad actors can try to steal your secret key. However, none of these are necessary so long as you are only using the cookie based technique for development testing. If you decide to use this in production, make sure the cookie is set as secure and as HttpOnly.