pallet-staking: properly call into try_state in tests

[kianenigma]

... instead of manually re-implementing some of the checks in mock.rs

substrate/frame/staking/src/mock.rs

Lines 550 to 555 in 20b5aac

	fn post_conditions() {
	check_nominators();
	check_exposures();
	check_ledgers();
	check_count();
	}

kinda related to #12130.

Ideally, in all pallets, we should call in that pallet's try-state at the end of each tests, but I am not sure if and how there is a way to enforce that, other than doing it manually, as we are doing for staking here.