[WIP] increase test coverage

[Rich-Harris]

Codecov has mysteriously started commenting on pull requests again, which reminds me that we should try and get our coverage a bit higher:

https://codecov.io/gh/sveltejs/svelte

[codecov-io]

Codecov Report

Merging #565 into master will increase coverage by 6.63%.
The diff coverage is 93.65%.

@@            Coverage Diff             @@
##           master     #565      +/-   ##
==========================================
+ Coverage   87.78%   94.42%   +6.63%     
==========================================
  Files          95       95              
  Lines        2866     2744     -122     
==========================================
+ Hits         2516     2591      +75     
+ Misses        350      153     -197

Impacted Files	Coverage Δ
src/generators/dom/visitors/Element/meta/Window.js	77.35% <ø> (+2.35%)	⬆️
src/generators/dom/index.js	97.56% <ø> (ø)	⬆️
src/validate/utils/FuzzySet.js	88.62% <ø> (ø)
src/validate/utils/list.js	100% <100%> (ø)
src/generators/dom/visitors/IfBlock.js	100% <100%> (+31.81%)	⬆️
src/validate/js/index.js	92.3% <100%> (-0.2%)	⬇️
src/validate/utils/fuzzymatch.js	100% <100%> (ø)
src/validate/js/propValidators/namespace.js	81.81% <100%> (-1.52%)	⬇️
src/validate/html/validateWindow.js	100% <100%> (ø)
src/validate/html/validateElement.js	78.12% <100%> (-4.49%)	⬇️
... and 19 more

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[Rich-Harris]

Alright, taking a break from this for a bit. Coverage is now almost at 95%, and I found and fixed a couple of bugs while going through this process