Migrate threaded reco ART tests
This MR switches the number of threads from 5 to 8 threads in RecJobTransformTests to reflect the proposed value in production jobs. It also adjusts five thread instances (and removes 2 thread instances) in RecExRecoTests. Many test files were renamed from "five" to "multiple" to better reflect that they in general test threaded behaviour and that the value would change in future in line with what is used in MC/data production.