𝓞 is extant here and reads “the holy scriptures testifies” rather than the expected “the holy scriptures testify”. Oliver Cowdery copied the third person singular testifies into 𝓟, but the 1830 typesetter changed it to testify. In his editing for the 1837 edition, Joseph Smith corrected 𝓟 to testify, thus making 𝓟 agree with the 1830 reading.
One possibility is that in the original text holy scriptures is acting as a single semantic unit here in Alma 34:30. There is actually one other example of this usage in the text:
In this instance, the 1830 typesetter retained the original “all the scriptures which gives an account”, but Joseph Smith made the change to the standard give in his editing for the 1837 edition. Otherwise, the plural form scriptures is consistently paired with plural verb forms (such as are, were, and have) or referred to by means of plural pronouns. There are no examples like “the scriptures is” or “the scriptures was”. The critical text, however, will restore the two instances where the associated verb for scriptures is a third person singular, here in Alma 34:30 and in 3 Nephi 28:33.
Summary: Restore in Alma 34:30 the third person singular present-tense form testifies (“the holy scriptures testifies of these things”); this is one of two instances in the text where scriptures acts like a singular and takes a singular verb form (the other one is in 3 Nephi 28:33).