Button test updates

[tomdye]

Replaces: #133
Resolves #135

[bitpshr]

Looks fine. Only comment I can think of is we could've used sinon for all the assertions that callback events are called, but there's not really any point.

Good to go.

[tomdye]

@bitpshr I'll raise an issue to revisit all unit tests to use sinon.

[tomdye]

@bitpshr issue raised: #234

[codecov]

Codecov Report

Merging #233 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #233      +/-   ##
==========================================
+ Coverage   99.14%   99.15%   +<.01%     
==========================================
  Files          22       22              
  Lines        1283     1295      +12     
  Branches      379      386       +7     
==========================================
+ Hits         1272     1284      +12     
  Partials       11       11

Impacted Files	Coverage Δ
src/button/Button.ts	100% <100%> (ø)	⬆️
src/checkbox/Checkbox.ts	100% <0%> (ø)	⬆️

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