xdsclient: deflake TestADS_ResourcesAreRequestedAfterStreamRestart

[easwars]

Reasons for the flake:

• The resource names requested from the management server were written on to a channel in a non-blocking fashion. This meant that the most recently requested names were not always available.
• A goroutine leak was also happening in some cases because the test was not reading the updates delivered to the resource watcher.

The fix addresses both of the above issues. It also improves the error message when the test times out waiting for a response that matches expectation.

Fixes #7714

#a71-xds-fallback
#xdsclient-refactor

RELEASE NOTES: none

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.96%. Comparing base (4115c21) to head (7df46ec).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7720      +/-   ##
==========================================
+ Coverage   81.78%   81.96%   +0.17%     
==========================================
  Files         361      361              
  Lines       27825    27825              
==========================================
+ Hits        22758    22807      +49     
+ Misses       3864     3829      -35     
+ Partials     1203     1189      -14     

see 22 files with indirect coverage changes