Fix TimetableSnapshot merge conflict

[vpaturet]

Summary

This PR fixes a merge conflict that occured when removing a setter method in TimetableSnapshot in one PR that was used in a unit test in another PR.
This PR refactors the unit test to create the test context method without using the setter method.

Issue

No

Unit tests

Fixed unit test

Documentation

No

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.63%. Comparing base (8de9f88) to head (e0f60f5).
Report is 5 commits behind head on dev-2.x.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #5972      +/-   ##
=============================================
- Coverage      69.63%   69.63%   -0.01%     
  Complexity     17129    17129              
=============================================
  Files           1937     1937              
  Lines          73742    73741       -1     
  Branches        7546     7546              
=============================================
- Hits           51351    51350       -1     
- Misses         19754    19755       +1     
+ Partials        2637     2636       -1     

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[vpaturet]

Side note: the test is marginally better since it goes through the same public API as an updater (at the cost of a more complex test setup)