test: Sync blocks in test_witness_block in p2p_segwit.py #36703

test_witness_block does not sync the blocks across nodes by default, so the test may intermittently fail.

Fixes #36700 by adding the sync.

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• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36703
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36703.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36703#author-ralph

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A CI run on FreeBSD is here: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/41051608081.

Ah, sorry, I forgot the diff to reproduce:

diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index c49b2fde40..b3ab04d65f 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -3688,2 +3688,3 @@ void PeerManagerImpl::ProcessBlock(CNode& node, const std::shared_ptr<const CBlo
 {
+    UninterruptibleSleep(60ms); // Temporary delay to reproduce block relay timing races.
     bool new_block{false};

crACK fab8fd026cfb24c9f246fb60fbff155f3414bacd

Post-merge ACK fab8fd026cfb24c9f246fb60fbff155f3414bacd.