Integration tests for staking + election-provider-multi-phase

[emostov]

There should be an integration test with at least session, election-provider-multi-phase, bags-list, and staking to document how the election emergency phase can resolved via intervention from Force::Origin.

In addition to catching a regression, this test should also serve as reference for how to handle an election emergency.

This can be a follow up to #8912

Originally posted by @emostov in #8912 (comment)

[kianenigma]

This, overall, is a call to add a staking-election-provider test crate. A test crate that tests the interactions of staking and pallet-election-provider-multi-phase in a realistic way. We can then test scenarios such as:

1. successful signed submission, unsigned submissions should not happen.
2. failed signed submission, unsigned submissions should help us recover.
3. both phases fail, emergency happens. using of the governance recovery approaches will bring everything back to normal.

[kianenigma]

Staking + EPM + Sessions + BagsList should allow us to test everything.

[kianenigma]

@gpestana this is probably a good place to start dumping any ideas around integration tests that we eventually want to take to versi as well.