A follow-up PR to simplify event assertions by introducing
`contains_event`, allowing event checks without needing exact field
matches. This reduces redundancy and makes tests more flexible.

Partially addresses #6119 by providing an alternative way to assert
events.

Reference: [PR #7594 -
Discussion](https://github.com/paritytech/polkadot-sdk/pull/7594#discussion_r1965566349)

---------

Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: Bastian Köcher <git@kchr.de>