Extract main() from example projects

[kmichaelk]

It will be possible to copy example without thinking about deleting main, and it will be easier to modify runners, for example, inject custom test listeners by modifying a single file.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.88%. Comparing base (fa242d1) to head (b1739f6).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
tasks/mpi/runner.cpp	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
- Coverage   96.96%   96.88%   -0.09%     
==========================================
  Files          28       33       +5     
  Lines         593      609      +16     
  Branches      173      178       +5     
==========================================
+ Hits          575      590      +15     
  Misses          2        2              
- Partials       16       17       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[allnes]

please add https://github.com/learning-process/ppc-2024-autumn/pull/618 in this PR

[kmichaelk]

IMO it is better to add it and https://github.com/learning-process/ppc-2024-autumn/pull/451 as separate patches, that's why I deduplicated the main first

[allnes]

@kmichaelk please move https://github.com/learning-process/ppc-2024-autumn/pull/618 this functionality