ELBv2: register_targets() now validates that instances are running

[bblommers]

Fixes #8286

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.58%. Comparing base (147049e) to head (a7edd5b).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8350   +/-   ##
=======================================
  Coverage   94.58%   94.58%           
=======================================
  Files        1159     1159           
  Lines      101119   101129   +10     
=======================================
+ Hits        95645    95655   +10     
  Misses       5474     5474           

Flag	Coverage Δ
servertests	28.90% <35.00%> (+<0.01%)	⬆️
unittests	94.55% <100.00%> (+<0.01%)	⬆️

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