settings: add another consistency test for SLUG_SUBSTITUTIONS.
settings: add another consistency test for SLUG_SUBSTITUTIONS.

getpelican/pelican,avaris/pelican,ingwinlu/pelican,iKevinY/pelican,iKevinY/pelican,getpelican/pelican,farseerfc/pelican,talha131/pelican,farseerfc/pelican,avaris/pelican,ingwinlu/pelican,talha131/pelican