This is not exactly what you are asking for, but there was an investigation into a rumored slave insurrection conspiracy in the 1740s that produced extensive slave testimony. The testimony was taken before the governor. I don't remember the exact year or what archive houses the transcript, but I believe you should be able to find the information in Robert Olwell's or Philip Morgan's book.
See Thomas D Morris, Southern Slavery and the Law, 1619-1860 (UNC Press, 1996), Chapter 10, "Slaves and the Rules of Evidence in Criminal Trials" (pp. 229-248) for detailed discussion of admissability of slaves' evidence from colonial days forward. Morris discusses specific colonies and states, including South Carolina.
You find the bulk of 18th and 19th century testimony of Virgina slaves and free blacks in the freedom suits filed in the chancery (equity) courts. I would suspect that South Carolina might have similar examples since they had chancery courts until 1868. While a bit different than direct testimony, following the Denmark Vesey conspiracy, the SC legislature passed a law requiring free blacks to petition the equity court with the testimony of a white "sponsor" who could vouch for their good character.
In North Carolina records I have seen first person accounts by slaves during the coroner's inquest stage of a murder investigation. I do not know if South Carolina has such a record series as coroner's inquests.
In North Carolina I have also seen a couple of slave courts dealing with murder committed by a slave that included testimony of a slave. If memory serves that record is in Northampton County records just prior to or at the outset of the Civil War (early 1861, is my recollection).
Sorry, my knowledge is limited to NC records.