Re-enabled VirtualizedList "retains batch render region when an item is appended" test

[chicio]

Summary:

This PR is a follow-up of #56358, in order to improve and VirtualizedList tests.
I re-enabled the skipped test retains batch render region when an item is appended.
With React 19, the previous approach that was using jest.runAllTimersAsync in this test path was unstable because the pre-update render region could still be processing updates.
I adopted the same approach used in #56358 by introducing a small helper function, advanceUntilLastCellIndexRendered, which advances timers one step at a time and stops when the expected state is reached: cellsAroundViewport.last === items.length - 1.

As part of this follow-up, I also aligned advanceUntilRenderAreaChanged to use performNextBatch for consistent stepwise timer advancement.

Changelog:

[GENERAL][FIXED] - Re-enabled VirtualizedList "retains batch render region when an item is appended" tes

Test Plan:

• Ran the VirtualizedList-test.js and verified all test cases passed.
• Verified test modification correctness by intentionally breaking the related snapshot and check that it is breaking/it is failing.