Ensure tests don't hang

[philippjfr]

Tests were hanging and it seems this was due to manually using the serve utility rather than using the more recent serve_and_wait and serve_and_request utilities.

I'm still a little confused at the difference since internally those utilities don't do anything too different except for having logic to wait until the server is actually up and running before continuing with the test.

The issue was that unless you explicitly provided a server_id to get_server the server_id in state._threads would not match the server_id on state._servers, meaning that when it came time to shut down the threads/servers the recent changes I made to the clean up logic would never find the thread to shut down.

[codecov]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 82.14%. Comparing base (5578f98) to head (75975b6).
Report is 1182 commits behind head on main.

Files	Patch %	Lines
panel/io/server.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6492      +/-   ##
==========================================
- Coverage   82.75%   82.14%   -0.61%     
==========================================
  Files         239      314      +75     
  Lines       34507    46313   +11806     
==========================================
+ Hits        28555    38046    +9491     
- Misses       5952     8267    +2315     

Flag	Coverage Δ
ui-tests	36.69% <88.00%> (-1.43%)	⬇️
unitexamples-tests	71.49% <96.00%> (-2.36%)	⬇️

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