chore: move tests into separate files

[RomainMuller]

Also chage coverage gathering mechanism again... Sorry for the churn. The new HTML report looks less good, but the line & region (~branch) coverage appear to be more reliable...

[codecov]

Codecov Report

Merging #132 (c17d445) into main (e129e9f) will increase coverage by 37.1%.
The diff coverage is 80.5%.

Additional details and impacted files

Components	Coverage Δ
Parser	75.4% <75.2%> (+35.9%)	⬆️
Intermediate Representation	78.4% <91.1%> (+39.6%)	⬆️
Synthesizers	60.2% <ø> (+27.8%)	⬆️
Other	59.4% <63.6%> (+37.4%)	⬆️

@@           Coverage Diff            @@
##            main    #132      +/-   ##
========================================
+ Coverage   34.0%   71.2%   +37.1%     
========================================
  Files         22      23       +1     
  Lines       2848    1900     -948     
  Branches    1348    1900     +552     
========================================
+ Hits         969    1352     +383     
+ Misses       880     249     -631     
+ Partials     999     299     -700     

Impacted Files	Coverage Δ
src/errors/mod.rs	91.7% <ø> (+32.6%)	⬆️
src/ir/conditions/mod.rs	65.0% <ø> (ø)
src/ir/importer/mod.rs	88.6% <ø> (ø)
src/ir/outputs/mod.rs	88.9% <0.0%> (ø)
src/ir/reference/mod.rs	61.7% <ø> (ø)
src/ir/resources/mod.rs	78.4% <ø> (ø)
src/lib.rs	50.0% <ø> (+41.5%)	⬆️
src/parser/condition/mod.rs	77.1% <ø> (ø)
src/parser/intrinsics/mod.rs	72.3% <ø> (ø)
src/parser/lookup_table/mod.rs	84.6% <ø> (ø)
... and 10 more

... and 2 files with indirect coverage changes

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