Do you really want a valid certificate for localhost?
I can get a valid certificate for development by simply getting a valid certificate for localhost.example.com on my server through let's encrypt and then making localhost.example.com resolve to 127.0.0.1 in my /etc/hosts file.
Some code can behave slightly differently on localhost than on localhost.example.com, for example in deciding whether to keep cookies on a third level domain, so tests are more reliable that way.
I can get a valid certificate for development by simply getting a valid certificate for localhost.example.com on my server through let's encrypt and then making localhost.example.com resolve to 127.0.0.1 in my /etc/hosts file.
Some code can behave slightly differently on localhost than on localhost.example.com, for example in deciding whether to keep cookies on a third level domain, so tests are more reliable that way.