editoast: refactor scenario V2 tests

[Wadjetz]

part of #6980

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.19231% with 5 lines in your changes missing coverage. Please review.

Project coverage is 28.47%. Comparing base (4b360d4) to head (0b1b79f).

Files	Patch %	Lines
editoast/src/views/v2/scenario.rs	90.19%	5 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #7668      +/-   ##
============================================
+ Coverage     28.45%   28.47%   +0.01%     
  Complexity     2038     2038              
============================================
  Files          1243     1243              
  Lines        153286   153344      +58     
  Branches       3008     3008              
============================================
+ Hits          43622    43668      +46     
- Misses       107873   107885      +12     
  Partials       1791     1791              

Flag	Coverage Δ
core	75.03% <ø> (ø)
editoast	71.89% <95.19%> (+0.01%)	⬆️
front	9.83% <ø> (ø)
gateway	2.41% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	72.93% <ø> (ø)

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[woshilapin]

Thank you! 🎉