[validitywindow] Unify Duplicate Implementations of Mock `ChainIndex`

[RodrigoVillar]

Currently, we have two implementations of a mock ChainIndex:

hypersdk/internal/validitywindow/validitywindow_test.go

Lines 427 to 429 in 2fada6d

	type testChainIndex struct {
	blocks map[ids.ID]ExecutionBlock[container]
	}

hypersdk/internal/validitywindow/validitywindowtest/test_chain_index.go

Lines 16 to 18 in 2fada6d

	type MockChainIndex[T emap.Item] struct {
	blocks map[ids.ID]validitywindow.ExecutionBlock[T]
	}

We should use a single implementation; this will most likely require us to split the validitywindow package into validitywindow and validitywindow_test.

[RodrigoVillar]

Ref: #1955 (comment)