test: fix sync function in rpc_psbt.py #32630

Even though the block is created on node2, the sync is only between node1 and node0. Accordingly the test fails if I put a sleep in msg_type == NetMsgType::HEADERS processing: In this case, node1 and node0 do not hear about the new block, the sync still passes because they are in sync with each other, and later on in the test_input_confs_control subtest, node1 would generate a forked block instead of building on the previous one, leading to test failure.

Haven't seen this in the CI, but I ran into it on an experimental branch.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32630.

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lgtm ACK 4df4df45d7bc2e8be99325d40cda936aab87c083

ACK 4df4df45d7bc2e8be99325d40cda936aab87c083