drm/i915/guc: Add a selftest for FAST_REQUEST errors

drm/i915/guc: Add a selftest for FAST_REQUEST errors

There is a mechanism for reporting errors from fire and forget H2G
messages. This is the only way to find out about almost any error in
the GuC backend submission path. So it would be useful to know that it
is working.

v2: Fix some dumb over-complications and a couple of typos - 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://patchwork.freedesktop.org/patch/msgid/20231114010016.234570-3-John.C.Harrison@Intel.com