improve parachain-staking test externalities

[4meta5]

What does it do?

adds a builder pattern for parachain-staking test externalities, instead of hiding the config behind functions

This is just like in #339 CC @JoshOrndorff

Improves readability

What important points reviewers should know?

Is there something left for follow-up PRs?

make genesis more readable #363 (comment)

What alternative implementations were considered?

Are there relevant PRs or issues in other repositories (Substrate, Polkadot, Frontier, Cumulus)?

What value does it bring to the blockchain users?

Checklist

• Does it require a purge of the network?
• You bumped the runtime version if there are breaking changes in the runtime ?
• Does it require changes in documentation/tutorials ?

[4meta5]

This makes me think I should change the parachain-staking genesis config from

pub struct GenesisConfig {
  ..
  stakers: Vec<(AccountId, Option<AccountId>, Balance)>
}

to

pub struct GenesisConfig {
  ..
  collators: Vec<(AccountId, Balance)>,
  nominators: Vec<(AccountId, AccountId, Balance)>,
}

This is more of a breaking change, but I do think it is objectively better.

[JoshOrndorff]

i think this is a good change.

The diff is huge mostly becaue of the tests.rs file. Is this just the result of indentation changing because of the new builder style? Or have the tests actually changed?

[4meta5]

Is this just the result of indentation changing because of the new builder style? Or have the tests actually changed?

Yes, the builder style caused lots of indentation changes because x_collators_y_nominators() functions were replaced with the specific ExtBuilder::default().with_collators().with_nominators().build()

And No, none of the tests changed at all otherwise