drm/xe/selftests: consider multi-GT for eviction test

We need to sanitize and reset each GT, since xe_bo_evict_all() will
evict everything regardless of GT, which can leave other GTs in a broken
state.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>