pallet-offences-benchmarking: Box events in verify

[bkchr]

Events in frame are represented by an enum in the pallet and the runtime. The size of an enum in Rust depends on the size of biggest variant. This means we always need to allocate memory for the biggest variant when allocating memory for an event. The offences benchmarking is verifying the benchmarking results by checking the events. To check the events it is generating all the expected events. With the recent changes in Polkadot the events are too big and lead to issues when running this verify functions. The solution is to box each event, as the vector holding all the events will then only need to hold fat pointers * expected events, instead of size_of(event) * expected events. This issue isn't a problem in production, as we never read the events on chain. When we are reading the events, it is done in an offchain context and they are only decoded one by one.

Besides that this also enables the benchmarking verification for everyone running these benchmarks.