drm/xe: Move test infra out of xe_pci.[ch]

Move code out of xe_pci.[ch] into tests/*.[ch], like is done in other
similar compilation units. Even if this is not part of "tests for
xe_pci.c", they are functions exported and required by other tests. It's
better not to clutter the module headers and sources with them.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230401085151.1786204-3-lucas.demarchi@intel.com