[improve: code coverage] ColorControl.js

[satotake]

related: https://github.com/times/cardkit/issues/61

Basically, this PR just added tests to ColorControl.js based on its original implementation,
I mean, it never adds any feature or functionality and is tiny improvement about coverage.
There is no real reason for starting with ColorControl.

Anyway...
Writing tests of ColorControl gave me 3 ideas.

1. it may be better for ColorControl to have a validator for layer prop.
   • ColorControl virtually requires layer.editable property but it is implicative.
2. Considered about the previous idea, it may be better for layer.editable to have consistency in terms of type.
   • For now, layer.editable[name] accepts object, string and boolean and it defines DOM content of ColorControl. I suspect this is complicated.
   • Just an idea
     • editable: {picker: 'circle', options: [...]} => CirclePicker
     • editable: {picker: 'chrome'} => ChromePicker
     • editable: {picker: 'input'} => input tag
3. ColorControl#handleChange should validate an argument.
   • Because it may not be a color-associated string.

If you are favor of even one of the ideas, I am willing to implement it (them) in next PR.
Thanks for reading.

[codecov-io]

Codecov Report

Merging #81 into master will increase coverage by 2.45%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   41.67%   44.13%   +2.45%     
==========================================
  Files          25       25              
  Lines        1310     1312       +2     
  Branches      250      251       +1     
==========================================
+ Hits          546      579      +33     
+ Misses        764      733      -31

Impacted Files	Coverage Δ
.../renderers/dom/ui/elements/Controls/SizeControl.js	34.88% <ø> (ø)	⬆️
...renderers/dom/ui/elements/Controls/ColorControl.js	100% <100%> (+60.86%)	⬆️
...nderers/dom/ui/elements/Panels/Layout/component.js	36.58% <0%> (ø)	⬆️
...rs/dom/ui/elements/Panels/Template/CardTemplate.js	37.93% <0%> (ø)	⬆️
src/renderers/shared/Card.js	13.52% <0%> (ø)	⬆️
...enderers/dom/ui/elements/Controls/SourceControl.js	22.8% <0%> (ø)	⬆️
...c/renderers/dom/ui/elements/Sidebar/PanelButton.js	42.85% <0%> (ø)	⬆️
src/renderers/dom/ui/elements/Canvas/component.js	65.21% <0%> (ø)	⬆️
src/renderers/dom/ui/ui.js	29.5% <0%> (ø)	⬆️
...erers/dom/ui/elements/Panels/Template/component.js	27.27% <0%> (ø)	⬆️
... and 8 more

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