Draft: Add dictionary for pair<string,unsigned long>>

Somehow gaudi/Gaudi!1412 (merged) exposes a problem of missing dictionary for pair<string,unsigned long>> in a few LHCb tests.

Adding the dictionary makes the warning message disappear and the test pass when gaudi/Gaudi!1412 (merged) is included.

(backport to run2-patches of !3938 (closed))