Section 8. In any criminal prosecution for the violation of any law relating to the listing of persons seventeen years of age and over and to the qualification or registration of voters, to voting lists or ballots or matters pertaining thereto, or to primaries, caucuses or elections or matters pertaining thereto, the presumption shall be that every proceeding or official act was valid, regular and formal; but evidence may be introduced either to rebut or support the presumption. As to an alleged election or primary, the testimony of the city or town clerk, and as to an alleged caucus, the testimony of the residing officer, secretary or clerk thereof, that such election, primary or caucus was held, shall be prima facie evidence that the same was regularly and duly held.