Confirm fork-awareness works in Availability Store Subsystem

[edwardmack]

Issue summary

The Polkadot Parachain Host Implementers' Guide defines what this subsystem should do, including being fork-aware, which is when there are multiple candidate instances proposed for a given block, when that block is finalized it must be handled, and the other candidates removed. The guide proposed the following test method:

• Fork-awareness of the relay chain is taken into account
  • Block B1 is added to the store.
  • Block B2 is added to the store.
  • Notify the subsystem that both B1 and B2 were included in different leafs of relay chain.
  • Notify the subsystem that the leaf with B1 was finalized.
  • Leaf with B2 is never finalized.
  • Leaf with B2 is pruned and its data is no longer available.
  • Wait until the finalized data of B1 should have been pruned.
  • B1 is no longer available.

Other information and links

[edwardmack]

Closed with PR #3896