test: import apport-retrace to get coverage data

[bdrung]

Calling apport-retrace as separate binary will prevent collecting code coverage data. So import the apport-retrace file and call the main function there.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 72.12%. Comparing base (24e599a) to head (6881b64).

Files with missing lines	Patch %	Lines
tests/system/test_apport_retrace.py	33.33%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #544       +/-   ##
===========================================
- Coverage   84.22%   72.12%   -12.11%     
===========================================
  Files         104      104               
  Lines       20792    20795        +3     
  Branches     3254     3255        +1     
===========================================
- Hits        17513    14998     -2515     
- Misses       2827     5384     +2557     
+ Partials      452      413       -39     

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