testutils: change ListenerWrapper to push the most recently accepted connection

[easwars]

#a71-xds-fallback
#xdsclient-refactor
Addresses #6902

Some e2e style tests cannot control the backoff behavior for reconnecting after a failed connection. So, making the ListenerWrapper push the most recently accepted connection on the channel, makes it easier for these tests.

RELEASE NOTES: none

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.37%. Comparing base (8212cf0) to head (8579907).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7772      +/-   ##
==========================================
- Coverage   81.40%   81.37%   -0.04%     
==========================================
  Files         368      368              
  Lines       36768    36768              
==========================================
- Hits        29932    29919      -13     
- Misses       5607     5617      +10     
- Partials     1229     1232       +3     

Files with missing lines	Coverage Δ
internal/testutils/wrappers.go	87.50% <100.00%> (ø)

... and 18 files with indirect coverage changes