I don't watch Corrie but how would you expect a police officer to test a substance for cocaine? Dipping his finger in it and then licking it, as happens in some poorly-researched police dramas, would be an almost certain way to kill him himself; only a laboratory test could suffice.
However there have plenty of real-life case where people have been caught in suspicious circumstances in possession of a white powder (which has later been shown to be nothing other than talc). It would be normal police procedure to arrest such a person "on suspicion of possession of a prohibited substance" (or something similar).