xds: Fix flaky test HandleListenerUpdate_ErrorUpdate

[arjan-bal]

Fixes: #8305

After #7977 the server's serving mode started being updated to NOT_SERVING every time a listener resource is NACKed. During the test, the xDS management server replies with the same resource version if a after client NACKs a resource. The test registers a callback that pushes the serving mode state updates to a buffered channel of size 1. The channel gets filled and blocks the test indefinitely.

This PR ensures we only keep the latest serving mode in the channel to avoid deadlocks. Verified that the test doesn't flake in 100k runs on forge.

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.37%. Comparing base (082a927) to head (5e063ba).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8397      +/-   ##
==========================================
+ Coverage   82.28%   82.37%   +0.08%     
==========================================
  Files         413      413              
  Lines       40454    40454              
==========================================
+ Hits        33289    33322      +33     
+ Misses       5795     5766      -29     
+ Partials     1370     1366       -4     

see 21 files with indirect coverage changes

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