test: add plugins and partial mixins unit test

[Phinome]

detail:

• add test of plugins
• add partial test of mixins
• update eslint to fix unit test

#433

[codecov]

Codecov Report

Merging #746 (2c8e81e) into d20 (37d5933) will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              d20     #746      +/-   ##
==========================================
- Coverage   76.10%   75.86%   -0.25%     
==========================================
  Files         181      182       +1     
  Lines        6421     6430       +9     
  Branches     1351     1351              
==========================================
- Hits         4887     4878       -9     
- Misses       1059     1077      +18     
  Partials      475      475              

Impacted Files	Coverage Δ
packages/veui/src/mixins/dropdown.js	36.00% <0.00%> (-24.00%)	⬇️
packages/veui/src/utils/dom.js	59.40% <0.00%> (-4.43%)	⬇️
packages/veui/src/mixins/key-select.js	91.37% <0.00%> (-1.73%)	⬇️
packages/veui/src/utils/plugin.js	100.00% <0.00%> (ø)
packages/veui/src/mixins/activatable.js	100.00% <0.00%> (+50.00%)	⬆️

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[Justineo]

@Phinome 如果 ready to merge 了，需要改一下 PR 状态，现在是 Draft。