[nextest-runner] move reporter events into their own file

[sunshowers]

These events are shared by all reporter code, so it makes sense to put them in their own file.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.52%. Comparing base (ea576d6) to head (4a13f32).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nextest-runner/src/reporter/events.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1908      +/-   ##
==========================================
- Coverage   79.53%   79.52%   -0.01%     
==========================================
  Files          80       81       +1     
  Lines       20575    20575              
==========================================
- Hits        16365    16363       -2     
- Misses       4210     4212       +2     

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