Here the 1888 LDS edition accidentally replaced the present-tense testifieth with the past-tense testified. This change was not transmitted into any subsequent LDS edition since the 1888 edition never served as a copytext for any other edition.
Although the past-tense form will work here in Helaman 13:26, usage earlier in this verse shows that the present-tense form is correct:
In fact, the larger passage (verses 26–28) expresses everything in the present tense. The critical text will maintain the present-tense testifieth both times here in verse 26.
Summary: Maintain the two instances of present-tense testifieth in Helaman 13:26, the reading of both 𝓟 and the 1830 edition.