xds: Add a test for incorrect load reporting when using pickfirst with servers in multiple localities

[arjan-bal]

This PR addressed the first task for #7339, adding a unit test that fails. The test presently asserts that the incorrect region received the load until the bug has been fixed:

The test asserts the following after sending 1 rpc:

1. Server 1 received the rpc
2. region-2 receives the load (wrong, but represents the current behaviour)

Then we stop server 1, send another rpc and assert the following:

1. Server 2 received the rpc
2. region-2 receives the load

Issue: #7339

RELEASE NOTES: None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.32%. Comparing base (98e5dee) to head (3f69df5).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7357      +/-   ##
==========================================
- Coverage   81.47%   81.32%   -0.16%     
==========================================
  Files         348      348              
  Lines       26761    26755       -6     
==========================================
- Hits        21804    21758      -46     
- Misses       3772     3794      +22     
- Partials     1185     1203      +18     

Files	Coverage Δ
internal/stubserver/stubserver.go	84.76% <100.00%> (+0.14%)	⬆️

... and 24 files with indirect coverage changes

[easwars]

Looks great now. Just some very minor nits.