test(Dropdown): add test for Dropdown

[eric-gitta-moore]

🤔 这个 PR 的性质是？

• 日常 bug 修复
• 新特性提交
• 文档改进
• 演示代码改进
• 组件样式/交互改进
• CI/CD 改进
• 重构
• 代码风格优化
• 测试用例
• 分支合并
• 其他

🔗 相关 Issue

#1418

💡 需求背景和解决方案

📝 更新日志

• test(Dropdown): Dropdown组件添加单元测试

• 本条 PR 不需要纳入 Changelog

☑️ 请求合并前的自查清单

⚠️ 请自检并全部勾选全部选项。⚠️

• 文档已补充或无须补充
• 代码演示已提供或无须提供
• TypeScript 定义已补充或无须补充
• Changelog 已提供或无须提供

[eric-gitta-moore]

@xiaosansiji

[github-actions]

[xiaosansiji]

老哥可以本地跑一下 npm run test:coverage，看看增加用例后 dropdown 的覆盖率嘛？

[codecov-commenter]

Codecov Report

Merging #1516 (cb0fdad) into develop (492d477) will increase coverage by 4.31%.
The diff coverage is 83.33%.

@@             Coverage Diff             @@
##           develop    #1516      +/-   ##
===========================================
+ Coverage    57.94%   62.26%   +4.31%     
===========================================
  Files          407      655     +248     
  Lines        15371    15822     +451     
  Branches      2931     2824     -107     
===========================================
+ Hits          8907     9851     +944     
+ Misses        6187     5679     -508     
- Partials       277      292      +15     

Impacted Files	Coverage Δ
src/alert/_usage/index.vue	0.00% <ø> (ø)
src/affix/affix.tsx	40.00% <80.00%> (+0.93%)	⬆️
src/alert/_example/icon.vue	100.00% <100.00%> (ø)
src/range-input/range-input-popup-props.ts	25.00% <0.00%> (-75.00%)	⬇️
src/grid/row-props.ts	71.42% <0.00%> (-28.58%)	⬇️
src/select/util.ts	65.00% <0.00%> (-26.67%)	⬇️
src/select/props.ts	54.54% <0.00%> (-20.46%)	⬇️
src/date-picker/props.ts	53.84% <0.00%> (-16.16%)	⬇️
src/date-picker/date-range-picker-props.ts	42.85% <0.00%> (-11.69%)	⬇️
src/select/optionGroup.tsx	31.57% <0.00%> (-9.60%)	⬇️
... and 440 more

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[eric-gitta-moore]

老哥可以本地跑一下 npm run test:coverage，看看增加用例后 dropdown 的覆盖率嘛？