core: stdcm: fix test for engineering allowance possibility

[eckter]

Fix some cases of #9023 (still not quite all of them)

The issue is that when computing engineering allowances, we compute the minimum time by computing a slowdown + speedup. Like this:

But because of a bug with the intersections, we computed this instead:

Which was way too permissive (reaching 0 means we can add infinite time there)

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 38.16%. Comparing base (d529bf2) to head (e8dabb3).
Report is 3 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #9903      +/-   ##
==========================================
- Coverage   38.18%   38.16%   -0.03%     
==========================================
  Files         997      997              
  Lines       92195    92195              
  Branches     1192     1192              
==========================================
- Hits        35209    35190      -19     
- Misses      56530    56549      +19     
  Partials      456      456              

Flag	Coverage Δ
editoast	73.33% <ø> (-0.07%)	⬇️
front	20.15% <ø> (ø)
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	87.00% <ø> (ø)

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