Set Payment Processor Test Mode from URL argument

1.0 Problem/Motivation
There is an option to set the Payment Processor into Test mode when configuring the Contribution. I have tried this with Stripe, and it's fine.

What I would like is the ability also to set Test mode from the URL, say with an argument such as '?contrib=test'. This would enable me to run regression tests for alterations made to the live site without having to take it out of service to change the configuration parameter, with the inconvenience and risk of error involved.

I am happy to have a go at producing a suitable patch, but have no idea where or how the setting of the configuration parameter is passed to Stripe. If someone could recommend a suitable place to try amending the code that would be very helpful.