Adds integration test for slashed/chilled validator with subsequent validation intention (#13717)

* Adds integration test for slashed/chilled validator with subsequent validation intention

* Removes unecessary comment

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

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

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

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Addresses PR review comments

* Fixes after conflict resolved

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Co-authored-by: parity-processbot <>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>