This PR fixes the broken request error handling of the workload filter
tests. Until now `validateHistoryPruneErr` was invoked with `fq.Err` as
an input which was always nil and a timeout or http error was reported
as a result content mismatch.
Also, in case of `errPrunedHistory` it is wrong to return here without
setting an error because then it will look like a valid empty result and
the check will later fail. So instead `errPrunedHistory` is always
returned now (without printing an error message) and the callers of
`run` should handle this special case (typically ignore silently).
