[Testing] Reduce testing time of measurement protocols

[chMoussa]

Solves #145 by:

• Refactoring tests to use several observables in shadows
• Reduce testing time of tomography. It turns out that QNN tests are a major bottleneck, especially a second-order cross-derivative. Also the zz_hamiltonian is quite long for the parameterized test test_forward_and_backward_passes_with_qnn. In my laptop, tests went from 5 minutes to 1 with these changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.53%. Comparing base (c426f17) to head (27d31bb).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #638   +/-   ##
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  Coverage   85.53%   85.53%           
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  Files         194      194           
  Lines       16663    16644   -19     
  Branches     1409     1410    +1     
=======================================
- Hits        14253    14237   -16     
+ Misses       2010     2007    -3     
  Partials      400      400           

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[jpmoutinho]

Thanks @chMoussa