MTCalibPeb: do some ROB prefetch calls in default options
After introducing random requests in MTCalibPebHypoTool (!22094 (merged)) and moving default options to all-random (!25699 (merged)), we could not prefetch and retrieve the same random ROBs in subsequent calls, which is what typically happens when running real HLT chains. This MR introduces an "ADDGET" instruction doing this and ensures we do some ROB prefetch calls in default options when running MTCalibPeb tests.
Due to the missing prefetch calls in the test, we missed some MT data access issues at P1 which were only discovered in the last Technical Run. With this change, it would be possible to reproduce the issues without full menu (but they were already resolved by Simone since then).