pickfirst: Stop test servers without closing listeners

[arjan-bal]

Fixes: #7868

In the above issue, the test failed with the following error log:

pickfirstleaf_ext_test.go:700: Failed to start server: net.Listen("tcp", "127.0.0.1:39795") = listen tcp 127.0.0.1:39795: bind: address already in use

In the test, the server was shut down and it couldn't bind to the same port when it tried to start again. This PR uses restartable listeners for the test server and exposes a methods on the test server to stop and restart the servers without closing the listeners.

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.82%. Comparing base (8b70aeb) to head (446264a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7872      +/-   ##
==========================================
- Coverage   82.04%   81.82%   -0.22%     
==========================================
  Files         377      377              
  Lines       38065    38065              
==========================================
- Hits        31229    31148      -81     
- Misses       5542     5604      +62     
- Partials     1294     1313      +19     

see 21 files with indirect coverage changes

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