drm/i915/selftests: Markup impossible error pointers

If we create a new live_context() we should have a mapping for each
engine. Document that assumption with an assertion.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190827094933.13778-1-chris@chris-wilson.co.uk