nomination-pools: Add era to Unbonded event

[wirednkod]

In nomination-pools tests that test cases that has Era as part of the case, it is a good idea to add in the assert_eq! of pool_events_since_last_call the era as part of the eq.
E.g. in the partial_unbond_max_chunks and this assert_eq! could have something like:

	Event::Unbonded { member: 20, pool_id: 1, balance: 3, points: 3, era: 1 },
	Event::Unbonded { member: 20, pool_id: 1, balance: 1, points: 1, era: 2 }

[hirschenberger]

So an EraIndex has to be added to Event::Unbonded. Does it make sense to add it to other events as well?

[kianenigma]

Which other events you have in mind @hirschenberger? It should be events that are era-related. Unbonding has a clear link to eras, because you will be unbnded 28 eras after. For, bonding for example doesn't have any significant semantical relation.

[hirschenberger]

It was just a question, I had no specific events in mind. I see that UnbondingPollSlashed already has an Era field, so I can add it to Unbonded and extend the tests.

[kianenigma]

Updated the title to clarify what is meant :)