core: tests: clean up stdcm unit tests

[eckter]

Two main changes:

1. Introduce a builder to make some parameters "optional" when calling STDCMPathfinding.findPath
2. Split the main test class into several new classes

There's a lot of diff but functionally nothing really changed. It will avoid editing dozens of tests each time we add new parameters.

If someone wants to proofread, focus on this commit. The other commit where I move some code around makes the diff mostly irrelevant.

[codecov]

Codecov Report

Merging #2622 (caedc17) into dev (4ea6b69) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                dev    #2622      +/-   ##
============================================
- Coverage     37.31%   37.29%   -0.02%     
+ Complexity     1679     1678       -1     
============================================
  Files           578      578              
  Lines         17598    17598              
  Branches       2435     2435              
============================================
- Hits           6566     6564       -2     
- Misses        10655    10656       +1     
- Partials        377      378       +1     

Flag	Coverage Δ
core	83.65% <ø> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...d/infra_state/implementation/TrainPathBuilder.java	81.94% <0.00%> (-1.39%)	⬇️

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