Are you asking us, the developers, how to test in development?
If so, most of us have taken to using Digital Ocean or Vultr for quickly spinning up a VM of each OS we support, and then running the installation script. Personally, I've also used Parallels to spin up VMs locally for testing.
We also all have Raspberry Pi's, which is where Pi-hole has it's roots, but testing is a bit more tedious there due to flashing the SD cards back to a clean state.
We encourage users to play and test in development via the pihole checkout command, with the understanding that it's not production-ready and is not always stable, but via that command, we have made it quite simple to switch back and forth between development and master branches.
It actually helps us a lot when we get feedback from our users. Since this is more technical, we typically keep the discussion over on GitHub, but we may consider making a new Development Feedback category here on discourse.pi-hole.net.