responder vs testee

responder and testee both are nouns.

Word	Noun	Adjective	Verb	Adverb
responder	Yes	No	No	No
testee	Yes	No	No	No

As nouns, testee is a hyponym of responder; that is, testee is a word with a more specific, narrower meaning than responder:

responder: someone who responds
testee: someone who is tested (as by an intelligence test or an academic examination)

Other hyponyms of responder include assenter, equivocator, hedger, tergiversator, interviewee, examinee.

responder (noun)	testee (noun)
someone who responds	someone who is tested (as by an intelligence test or an academic examination)