Repair add as a child

[valer1435]

Issues with _add_as_child mutation were solved. Also added tests for checking correctness of default mutations

[codecov[bot]]

Codecov Report

Merging #923 (c1f4b69) into master (2091c36) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #923   +/-   ##
=======================================
  Coverage   88.25%   88.26%           
=======================================
  Files         205      205           
  Lines       13415    13416    +1     
=======================================
+ Hits        11840    11841    +1     
  Misses       1575     1575           

Impacted Files	Coverage Δ
...edot/core/optimisers/gp_comp/operators/mutation.py	93.88% <100.00%> (+0.46%)	:arrow_up:
...ot/core/composer/gp_composer/specific_operators.py	88.13% <0.00%> (-3.39%)	:arrow_down:
fedot/core/pipelines/verification_rules.py	97.41% <0.00%> (+0.64%)	:arrow_up:

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