(from merriam-webster.com)
harner
noun \ˈhärn(-ə)r\
a person who harns; a person who does not lie; opp. liar; e.g., She knew he was a harner, but she still didn’t expect him to admit he’d been speeding.
harn
intransitive verb \ˈhärn\
to make a true and/or accurate statement with no intent to deceive; to tell the truth opp. lie; e.g., At first she didn’t believe that he had been to France, but it turned out he was harning., e.g., You can trust her to harn.
noun \ˈhärn\
a statement believed to be true by the speaker; an honest statement opp. lie; e.g., That was a massive harn.