Add Tests for Staking Payouts after Historic Support implementation

[Dominique]

Proposed Change or Idea
The rapid implementation of #1397 was needed by urgent requirements so no tests were included initially. It is important to add proper tests to ensure that the /accounts/{accountId}/staking-payouts endpoint works as expected after the historic support implementation. That way in the future we can avoid more fixes like #1415 and #1422

Example tests

• Staking Payouts in Kusama chain for :
  • eras < 518
  • eras right in the change of 518 (special cases)
  • era = 0
• Staking Payouts in Polkadot (or another chain) chain to ensure that general functionality (not related to historic support) is not affected
• Tests that combine the available query parameters: era, depth, unclaimedOnly.