fix: fix flaky calling_sync_routing_table by capturing timestamp earlier

[okekefrancis112]

This PR fixes flakiness in the calling_sync_routing_table test in lightning/src/routing/gossip.rs under CI load by capturing expected_timestamp before calling peer_connected(). Previously, the test captured the timestamp after the call, so under load any elapsed time could cause assertions comparing msg.first_timestamp against expected_timestamp to fail. Moving the capture earlier ensures expected_timestamp <= msg.first_timestamp, making the assertions deterministic while leaving test logic unchanged. This change addresses intermittent CI failures without affecting functionality.

Closes: #3875

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.02%. Comparing base (2e4a2ac) to head (38a17ee).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4399      +/-   ##
==========================================
- Coverage   86.02%   86.02%   -0.01%     
==========================================
  Files         156      156              
  Lines      103100   103100              
  Branches   103100   103100              
==========================================
- Hits        88693    88690       -3     
  Misses      11896    11896              
- Partials     2511     2514       +3     

Flag	Coverage Δ
tests	86.02% <100.00%> (-0.01%)	⬇️

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