Better handling of special property types in DAQSys.decoder-compare

this fixes a problem that !2676 (merged) didn't prevent

added all-slots modernisation testing labels

• [2020-10-14 00:04] Validation started with lhcb-run2-patches-dev4#480
• [2020-10-14 00:10] Validation started with lhcb-run2-patches#838

removed all-slots label

merged

mentioned in commit 4cd9c07f