test: wait for the node to finish the last block in p2p_ping_ibd #36687

ProcessBlock() stamps m_last_block_time only after ProcessNewBlock() returns, so getblockcount can show the new height, with nothing in flight, before the stamp is taken. A bumpmocktime in that gap stamps the block with the bumped time, and the final bump then leaves the ping timeout exactly at its boundary, where it never fires. Sync with a ping after the last block, so the node has finished processing it before the clock moves.

The race has been present since the test was added in #36080.

Faling job: https://github.com/bitcoin/bitcoin/actions/runs/40589303246/job/122489039018

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36687
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36687.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36687#author-ralph

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