Add event tests to Nomination Pools (#11349)

* fix a few things with nomination pools

* fix typo

* fix build

* eventify tests

* Update frame/nomination-pools/src/tests.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* fmt

* comments

* split

Co-authored-by: kianenigma <kian@parity.io>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>