Integration tests for staking + election-provider-multi-phase (paritytech#12972)

* EPM and staking pallets: Adds new crate for integration tests

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* Adds ExtBuilder and helpers with initial conditions assertions

* removes account helpers; adds staking, session, etc genesis

* Adds kusama incident test case

* Prepare for slashing test

* Adds solution submission

* slash_through_offending_threshold

* Renames e2e integration tests dir and crate

* consistently slash 10% of validator set

* finishes continous_slashes_below_offending_threshold test

* Update frame/election-provider-multi-phase/test-staking-e2e/src/lib.rs

Co-authored-by: Ankan <[email protected]>

* Update frame/election-provider-multi-phase/test-staking-e2e/src/lib.rs

Co-authored-by: Ankan <[email protected]>

* Update frame/election-provider-multi-phase/test-staking-e2e/src/lib.rs

Co-authored-by: Kian Paimani <[email protected]>

* Update frame/election-provider-multi-phase/test-staking-e2e/src/mock.rs

Co-authored-by: Kian Paimani <[email protected]>

* Update frame/election-provider-multi-phase/test-staking-e2e/src/mock.rs

Co-authored-by: Kian Paimani <[email protected]>

* Update frame/election-provider-multi-phase/test-staking-e2e/src/mock.rs

Co-authored-by: Kian Paimani <[email protected]>

* mock fixes

* Additional checks to delayed solution eras and mock fixes

* nits and addresses review comments; splits ext_builder into one per pallet

* helper to set balances ext builder

* bring up mock.rs to master

* integration test fixes and additions

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Co-authored-by: Ankan <[email protected]>
Co-authored-by: Kian Paimani <[email protected]>

Author: 3 people