[feat] Skip by default flexible tests when not enough nodes found

[vkarak]

Flexible tests are now by default skipped when not enough nodes are found. We introduce a new option, --flex-alloc-strict that will cause the test to fail instead (the current default).

Closes #3164.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.67%. Comparing base (9607459) to head (6f6aba0).
Report is 12 commits behind head on develop.

❗ Current head 6f6aba0 differs from pull request most recent head 074f39f. Consider uploading reports for the commit 074f39f to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3173   +/-   ##
========================================
  Coverage    86.67%   86.67%           
========================================
  Files           61       61           
  Lines        12139    12143    +4     
========================================
+ Hits         10521    10525    +4     
  Misses        1618     1618           

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