As nouns the difference between tester and yester
is that
tester is a canopy over a bed or
tester can be a person who administers a test or
tester can be an old french silver coin while
yester is (archaic) yesterday.
As an adverb yester is
(archaic) yesterday.
As an adjective yester is
(archaic) last; last past; next before; of or pertaining to yesterday.