Allow specifying whether JIT is enabled.

pyperf has different warmup policies for JIT and non-JIT implementations, and for instance -- if a new python implementation is under development for some research purpose: it introduces JIT and does not want to name itself pypy or graalpython, but pyperf will not detect it as an implementation with JIT.

https://github.com/psf/pyperf/blob/main/pyperf/_utils.py#L204