drm/xe/guc: Add test for G2G communications

Add a test for sending messages from every GuC to every other GuC to
test G2G communications.

Note that, being a debug only feature, the test interface only exists
in pre-production builds of the GuC firmware.

v2: Fix 'default' case to actually use the driver's registration code
as well as allocation. Add comments explaining the different test
types. Fix (C) date and an assert. Review feedback from Daniele.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://lore.kernel.org/r/20250910210237.603576-5-John.C.Harrison@Intel.com