Cleanup classes handling Events

[grospelliergilles]

• Use googlest to test theses classes
• Prevent copy or assignment for EventObservableBase
• Make some functions inline.

[codecov]

Codecov Report

Attention: Patch coverage is 95.08197% with 3 lines in your changes missing coverage. Please review.

Project coverage is 70.60%. Comparing base (b7480cb) to head (98f1c03).
Report is 5 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
arcane/src/arcane/utils/Event.cc	81.81%	0 Missing and 2 partials ⚠️
arcane/src/arcane/utils/tests/TestEvent.cc	97.61%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2010   +/-   ##
=======================================
  Coverage   70.60%   70.60%           
=======================================
  Files        2288     2289    +1     
  Lines      167469   167466    -3     
  Branches    19291    19290    -1     
=======================================
+ Hits       118236   118241    +5     
+ Misses      42149    42145    -4     
+ Partials     7084     7080    -4     

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