ci: compile header only tests in Debug mode

[zonca]

The header only tests were compiled in Release mode,
therefore making all assert statements ineffective.

See for example zonca#4
where I add an error in the unit test but tests pass anyway

[zonca]

after this fix, the test where I added an error actually fails:
zonca#5

[codecov]

Codecov Report

Merging #2870 (3190cd6) into main (123fa09) will increase coverage by 0.05%.
Report is 1 commits behind head on main.
The diff coverage is 77.88%.

Additional details and impacted files

Files	Coverage Δ
src/awkward/contents/bitmaskedarray.py	70.81% <100.00%> (+1.53%)	⬆️
src/awkward/contents/bytemaskedarray.py	89.75% <100.00%> (+0.73%)	⬆️
src/awkward/contents/content.py	74.81% <100.00%> (-0.28%)	⬇️
src/awkward/contents/listoffsetarray.py	82.86% <100.00%> (+0.13%)	⬆️
src/awkward/contents/numpyarray.py	91.18% <100.00%> (ø)
src/awkward/contents/regulararray.py	87.43% <100.00%> (+0.18%)	⬆️
src/awkward/contents/unmaskedarray.py	74.60% <100.00%> (+0.39%)	⬆️
src/awkward/contents/listarray.py	91.78% <83.33%> (+1.06%)	⬆️
src/awkward/contents/unionarray.py	85.22% <75.00%> (-0.25%)	⬇️
src/awkward/contents/indexedarray.py	80.95% <82.35%> (+2.05%)	⬆️
... and 2 more

... and 1 file with indirect coverage changes

[ianna]

@zonca - yes, indeed, the assertions are compiled out in a release build. Thanks for catching it!

[agoose77]

Thanks @zonca - this was a good oversight to catch!