e2e test for stunnel

[alaypatel07]

Describe the feature you'd like to have.
As part of the discussion from #4, it was recommended that ideally we should be able to use the configuration parameters for stunnel config (both server and client) and verify that when they are setup, they always authenticate with each other using the certs, and by extension, they are alway setup with the parameters we expect. This could be imagined being implemented as controller-runtime envtests which installs and runs the control-plane binaries.

What is the value to the end user? (why is it a priority?)
This will make sure that the library always provides a valid configuration and prevent newer development work breaking existing configuration options.