Moved the tests in the respective submodule subdirectories

[cosimoNigro]

This PR moves the tests in the respective submodules subdirectories.
Test files are becoming larger and larger and they were accumulating in the same subdirectory.
We can now split them if we mean to test different classes (see e.g. what already done in the synchrotron and fit submodules).
Solves #146.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c3be0a3) 97.02% compared to head (a115321) 97.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
- Coverage   97.02%   97.02%   -0.01%     
==========================================
  Files          43       43              
  Lines        3395     3394       -1     
==========================================
- Hits         3294     3293       -1     
  Misses        101      101              

Flag	Coverage Δ
unittests	97.02% <100.00%> (-0.01%)	⬇️

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