Disable back-tracking in PU pre-tracking for FastChain ART test and increase cores to 8
This MR disables back-tracking in PU pre-tracking for FastChain ART tests, following MR !75834 (merged) where the TRT overlay processes standard TRT_RDOs during track overlay. Additionally, it increases the number of cores to 8 in FastChain ART tests running in MP/MT mode to prevent memory failures. This change aligns with standard configurations for production jobs.