chore: fix coverage reporting for tests

[Trott]

tap is not reporting coverage, probably due to the switch to ESM.

Switch to c8.

[Trott]

Ah, I see I need to generate coverage data for codecov.io and upload it. Shouldn't be a problem, but have to step away from this for a bit.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.76 🎉

Comparison is base (e172a82) 93.93% compared to head (4bb187a) 94.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
+ Coverage   93.93%   94.70%   +0.76%     
==========================================
  Files          19       18       -1     
  Lines         462     1302     +840     
==========================================
+ Hits          434     1233     +799     
- Misses         28       69      +41     

Impacted Files	Coverage Δ
bin/cmd.js	91.92% <100.00%> (+1.22%)	⬆️
index.js	100.00% <100.00%> (ø)
lib/format-pretty.js	87.20% <100.00%> (+3.87%)	⬆️
lib/format-tap.js	58.10% <100.00%> (-8.56%)	⬇️
lib/rule.js	100.00% <100.00%> (ø)
lib/rules/co-authored-by-is-trailer.js	100.00% <100.00%> (ø)
lib/rules/fixes-url.js	94.79% <100.00%> (-1.51%)	⬇️
lib/rules/line-after-title.js	100.00% <100.00%> (ø)
lib/rules/line-length.js	100.00% <100.00%> (ø)
lib/rules/metadata-end.js	100.00% <100.00%> (ø)
... and 8 more

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[Trott]

There we go. Now it's working.