Add minor improvements to `chill_other` test (#7553)

# Description

https://github.com/open-web3-stack/polkadot-ecosystem-tests/pull/174
showed the test for the `pallet_staking::chill_other` extrinsic could be
more exhaustive.

This PR adds those checks, and also a few more to another test related
to `chill_other`,
`pallet_staking::tests::change_of_absolute_max_nominations`.

## Integration

N/A

## Review Notes

N/A

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Co-authored-by: Bastian Köcher <git@kchr.de>