test: cover disconnect on private broadcast peer with relay=false #35681

Currently, there is no test case to verify the disconnection of a private broadcast connection when the peer does not support transaction relay. This PR addresses it.

Can be tested with:

diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index 27f0a63c55..5d9a8f9c06 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -3741,7 +3741,7 @@ void PeerManagerImpl::ProcessMessage(Peer& peer, CNode& pfrom, const std::string
             } else {
                 LogDebug(BCLog::PRIVBROADCAST, "Disconnecting: does not support transaction relay (connected in vain), %s",
                          pfrom.LogPeer());
-                pfrom.fDisconnect = true;
+                pfrom.fDisconnect = false;
             }
             return;
         }

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35681.

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline and AI policy for information on the review process. A summary of reviews will appear here.

<!--5faf32d7da4f0f540f40219e4f7537a3-->