Make ./runtest --dump-logs dump logs for timeout tests's servers

[yzc-yzc]

Kill the servers first, then kill the test clients, so that we can dump logs for timeout tests's servers easily.
Note: Technically, we need to wait a while after killing the servers. But in practice I think it's enough now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.55%. Comparing base (1fbf5fb) to head (2472945).
⚠️ Report is 1 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #2412      +/-   ##
============================================
- Coverage     71.55%   71.55%   -0.01%     
============================================
  Files           123      123              
  Lines         67453    67453              
============================================
- Hits          48268    48263       -5     
- Misses        19185    19190       +5     

see 14 files with indirect coverage changes

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