xds: switching to stubserver in tests instead of testservice implementation

[janardhanvissa]

Partially Addresses: https://github.com/grpc/grpc-go/issues/7291

RELEASE NOTES: None

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.66%. Comparing base (d66fc3a) to head (e5a21ea). Report is 22 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7726      +/-   ##
==========================================
- Coverage   81.71%   81.66%   -0.05%     
==========================================
  Files         374      374              
  Lines       38166    37978     -188     
==========================================
- Hits        31188    31016     -172     
+ Misses       5699     5643      -56     
- Partials     1279     1319      +40     

see 52 files with indirect coverage changes

[purnesh42H]

@easwars for second review

[arvindbr8]

@janardhankrishna-sai -- Also please mention the PR description why does this diff "partially" fix the issue. The last few comments in https://github.com/grpc/grpc-go/issues/7291 is also not very clear.

[janardhanvissa]

@janardhankrishna-sai -- Also please mention the PR description why does this diff "partially" fix the issue. The last few comments in #7291 is also not very clear.