The function of such tests was to invoke the grandfather clause

Are you sure? The Slate article did not mention that as the function. According to the test:

This test is to be given to anyone who cannot prove a fifth grade education.

The Slate article doesn't mention grandfather clauses either, instead saying:

The literacy test—supposedly applicable to both white and black prospective voters who couldn’t prove a certain level of education but in actuality disproportionately administered to black voters—was a classic example of one of these barriers.