[backend/tests] Import/export of simulations, JSON + Excel

[antoinemzs]

Proposed changes

Related issues

• Closes #ISSUE-NUMBER

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality
• For bug fix -> I implemented a test that covers the bug

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

Attention: Patch coverage is 61.72840% with 62 lines in your changes missing coverage. Please review.

Project coverage is 33.32%. Comparing base (5fe4b1a) to head (14b5264).
Report is 1 commits behind head on release/1.11.0.

Files with missing lines	Patch %	Lines
...nbas/rest/exercise/exports/ExerciseFileExport.java	56.64%	32 Missing and 30 partials ⚠️

Additional details and impacted files

@@                 Coverage Diff                  @@
##             release/1.11.0    #2156      +/-   ##
====================================================
+ Coverage             32.59%   33.32%   +0.73%     
- Complexity             1514     1555      +41     
====================================================
  Files                   582      583       +1     
  Lines                 18091    18154      +63     
  Branches               1164     1178      +14     
====================================================
+ Hits                   5896     6050     +154     
+ Misses                11931    11805     -126     
- Partials                264      299      +35     

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