What's the best tool for developers to A/B test?

I was often looking for a simple tool that does exceptional at A/B testing that I can use as a developer. I want to create a test, plug some API calls into my website and get the result.

All the tools I've encountered so far are way too marketing related (although I guess it's mainly used for marketing) and are half-baked website builders and work with modifying JavaScript.

Any suggestions?