stats: replace testServer with StubServer in stats_test.go

[110y]

• stats: replace testServer with StubServer in stats_test.go
  • stubserver: add support to optionally pass in a grpc.Server or xds.GRPCServer #7291

RELEASE NOTES: none

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.09%. Comparing base (36e13de) to head (bb47fbb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8980      +/-   ##
==========================================
+ Coverage   83.01%   83.09%   +0.07%     
==========================================
  Files         411      411              
  Lines       32719    32719              
==========================================
+ Hits        27162    27187      +25     
+ Misses       4167     4148      -19     
+ Partials     1390     1384       -6     

see 29 files with indirect coverage changes

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[easwars]

Moving to @mbissa for second set of eyes.