Slow running tests should not show `RejectList`ed packages

Slow running tests could do with an overhaul. What I miss most there is grouping per source package and a way to see only the > 2:47 tests. If I remember correctly, in one of your earlier tasks we already enabled the time to be a configuration item (server-side), so that can be fixed already. The page should also filter out results for packages that are on the RejectList for the particular suite/architecture (I think globally RejectListed packages are already filtered out.