Fix molecule integration test

[YamLyubov]

The problem was with get_structure_unique_population. It was extending population using mutation (EvoGraphOptimizer._extend_population) but added molecules were not evaluated. That is why individuals with null fitness appeared in the population.

144

Also the check optimized_network.depth > 1 from test_custom was removed because it can find cycled graphs with depth == -1

[codecov-commenter]

Codecov Report

Merging #145 (f36b6f8) into main (f64661f) will decrease coverage by 0.21%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
- Coverage   73.81%   73.61%   -0.21%     
==========================================
  Files         129      129              
  Lines        7563     7566       +3     
==========================================
- Hits         5583     5570      -13     
- Misses       1980     1996      +16     

Impacted Files	Coverage Δ
golem/core/optimisers/populational_optimizer.py	92.15% <100.00%> (-3.89%)	:arrow_down:

... and 9 files with indirect coverage changes