An old French silver coin, originally of the value of abouteighteen pence, subsequently reduced to ninepence, and later tosixpence, sterling. Hence, in modern English slang, a sixpence; --often contracted to tizzy. Called also teston. Shak.
When an illegal drug crew gets a new shipment and/or a new supplier, it is not uncommon for them to have one day where they give out testers, essentially free samples that junkies will ultimately comment on the quality of. It ensures the dealers or not being ripped off by suppliers and can also cause some junkies to become loyal to that crew and their product. Junkies are often very excited for tester day, since it means a rare free high.
ever seen a fat chav bloke with shitloads of tatoos? well hes probably a tester. tatoo parlours grab random blokes off the street, drug em then draw random shit on them.
* These videos are coming directly from Youtube, they may or may not be most relevant to the word "Tester"