Green’s Dictionary of Slang

tester n.2

(US drugs) a free sample of a given drug, given out so as to encourage word-of-mouth reports that the dealer’s stock is worth buying.

[US]Simon & Burns Corner (1998) 115: In every open-air market in the city, samples are offered up early in the day to spead the word that so-and-so’s shit is truly a bomb. And because a weak tester would be self-defeating, the free samples rarely disappoint.
[US]Simon & Burns ‘The Target’ Wire ep. 1 [TV script] You put out testers?
[US]Simon & Burns ‘–30–’ Wire ser. 5 ep. 10 [TV script] They gonna put out testers on Poplar and Grove.