Fix coverage reporting

[lefticus]

• Need to make sure opt_enable_coverage is set when appropriate (only for Debug builds, since it overrides optimization settings)
• Remove explicit sources dir from windows coverage, so that it still works when template project is used
• Make sure gcov version/executable is set correctly
• Add simple test of main executable, to show its coverage also

Note: these issues were discovered while using a recent project made from the template. I think most of these issues are long standing. Previously ENABLE_COVERAGE was enabled by default, so it happened to work, but Release builds were not doing what we thought they were doing.

[codecov-commenter]

Codecov Report

Merging #191 (1bc4c99) into main (1e56cae) will decrease coverage by 20.00%. The diff coverage is n/a.

@@             Coverage Diff              @@
##              main     #191       +/-   ##
============================================
- Coverage   100.00%   80.00%   -20.00%     
============================================
  Files            2        3        +1     
  Lines           20       35       +15     
  Branches         0       18       +18     
============================================
+ Hits            20       28        +8     
- Misses           0        6        +6     
- Partials         0        1        +1     

Flag	Coverage Δ
Linux	38.46% <ø> (∅)
Windows	80.00% <ø> (-20.00%)	:arrow_down:
macOS	38.46% <ø> (∅)

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Impacted Files	Coverage Δ
test/tests.cpp	100.00% <0.00%> (ø)
test/constexpr_tests.cpp	100.00% <0.00%> (ø)
src/main.cpp	46.15% <0.00%> (ø)

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