You could say 'bear witness' which is the opposite of 'not bear'bear false witness', prefixed in the Ten Commandments with 'thou shalt not'.
As a side note I think it stems from the difference in the nature of the words. As truth is viewed as singular as there is only one truth (from one person's perspective anyway) but there are many lies possible as an alternative to that truth. It's very difficult to provide an antonym of a plural which is by nature singular.