Add doctest support into coverage

[awong-dev]

Moving tests to doctests caused the previous llvm-cov command to stop running them. Adding the --doctest flag puts them back in but requires the nightly toolchain.

[codspeed-hq]

CodSpeed Performance Report

Merging #105 will not alter performance

_{Comparing awong-dev:coverage (ab1eab3) with main (e5129f0)}

Summary

✅ 10 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.95%. Comparing base (e5129f0) to head (ab1eab3).

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #105       +/-   ##
===========================================
+ Coverage   70.44%   82.95%   +12.51%     
===========================================
  Files           8        8               
  Lines        1800     1960      +160     
===========================================
+ Hits         1268     1626      +358     
+ Misses        532      334      -198     

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[awong-dev]

@messense: This one should be pretty straightforward. It fixes the coverage regression I introduced accidentally in #100.