[Umbrella ticket] Improve try-state and integrity-tests coverage for EPM-MB pallet and sub-pallets

[gpestana]

Improve the coverage of try-state checks for the EPM-MB and its sub-pallets. In addition, the new EPM-MB and its sub-pallets have a few new config invariants that can and should be checked by the integrity_test hook.

The EPM-MB pallet and sub-pallets haven't been merged to master yet, check #6034.

Note for mentees: each of the tasks of this umbrella ticket is a good mentor task for who wants to help with the multi-block election/staking effort. All the storage invariants should be documented in the pallet's rustdoc, otherwise ping @gpestana.

Tasks

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1. try-state / integrity-test checks for EPM-MB core
2. try-state / integrity-test checks for signed pallet
3. try-state / integrity-test checks for unsigned pallet
4. try-state / integrity-test checks for verifier pallet

[Krayt78]

Hi @gpestana along with @ndkazu and @Cjumelin we want to tackle this issue :)