Improve has_realtime_kernel method (backport #260)

[mergify]

These changes allow the detection of realtime kernel even from the uname output. It should be more robust.

Fixes: #257

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This is an automatic backport of pull request #260 done by Mergify.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 72.40%. Comparing base (4c64752) to head (dab7de4).
Report is 1 commits behind head on humble.

Files with missing lines	Patch %	Lines
src/realtime_helpers.cpp	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           humble     #267      +/-   ##
==========================================
- Coverage   73.45%   72.40%   -1.06%     
==========================================
  Files           9        9              
  Lines         550      558       +8     
  Branches      215      220       +5     
==========================================
  Hits          404      404              
- Misses         96      104       +8     
  Partials       50       50              

Flag	Coverage Δ
unittests	72.40% <0.00%> (-1.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/realtime_helpers.cpp	41.07% <0.00%> (-3.16%)	⬇️