Yes, you could (theoretically) use an emulator for the microcontroller. I tried it a few times. And it isn’t fun. It takes a lot of effort to simulate all inputs, outputs are hard to interpret and all kind of effects of the real light aren’t reproduced.

In reality we flash the build onto a flashlight and try it. Sometimes it’s only a prototype, disassembled. And rarely it’s an actual devboard which has all relevant parts of the flashlight nicely accessible (basically the microcontroller with required electronics, regulated power supply, low power LED to see the output, several LEDs for aux and button and the switch itself).