[SYCL, CI] add a SYCL CPU test harness

[abagusetty]

Check if default_selector or cpu_selector can be used for CPU only CI testing

[wavefunction91]

@abagusetty Something to this effect has been added in #30, take a look?

[abagusetty]

You might want to consider switching default_selector_v to cpu_selector_v explictly, since the default ranking would choose the GPUs there, if/when present.
https://github.com/wavefunction91/ExchCXX/pull/30/files#diff-6cd7a23ffaff44f8dea0936ed90ca0463d6ec54b6e49c16acb0bc67b51efd55eR1947