[NPoS] Use EraInfo to manipulate exposure in fast-unstake tests (#2459)

The FastUnstake pallet tests were previously directly modifying storage
items in the pallet-staking to alter exposure. However, due to the
introduction of the [new paged exposure
feature](https://github.com/paritytech/polkadot-sdk/pull/1189) these
tests were not testing against correct storage items. This issue
resulted in a bug that I didn't catch, which has been addressed in [this
fix](https://github.com/paritytech/polkadot-sdk/pull/2369).

This PR introduces a modification to how the pallet-fast-unstake handles
exposure. It now utilizes `pallet-staking::EraInfo` to set or mutate
Exposures.