Implements `try_state` hook in elections and EPM pallets (#13979)

* Adds try_state hook to elections pallets

* Addresses PR review comments

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

* remove unecessary println

* ensures try-runtime does not mutate storage

* Addresses PR comments

* Fixes snapshot invariant checks; simplifies test infra

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Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: parity-processbot <>