GPU emulator

**Is your feature request related to a problem? Please describe**

It would be great if one could emulate a GPU (like there is an FPGA emulator) purely for correctness testing rather than for performance (I would naively expect performance to be very bad). The use case I have in mind is for testing in CI (e.g. using GitHub actions) where we do not typically have access to GPUs (unless we shift to hosting ourselves).

**Describe the solution you would like**

The creation of a GPU emulator like there is for FPGAs.

**Describe alternatives you have considered**

For SYCL, I guess one could just test the CPU device but this does not fully test correct behavior on GPUs. The other alternative is just testing on physical hardware but this is not always feasible.

**Additional context**

I understand this is a rather large feature request so don't expect anything soon but thought I'd create the issue following discussions with @Pennycook.