chore: improve webpack 5 initial test

[sorrycc]

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• any feature?
• close https://github.com/umijs/umi/ISSUE_URL

[codecov]

Codecov Report

Merging #6493 (1c6b064) into master (0867356) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #6493      +/-   ##
==========================================
- Coverage   84.76%   84.71%   -0.05%     
==========================================
  Files         157      157              
  Lines        3583     3585       +2     
  Branches      986      989       +3     
==========================================
  Hits         3037     3037              
- Misses        536      538       +2     
  Partials       10       10              

Impacted Files	Coverage Δ
packages/umi/src/initWebpack.ts	0.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0867356...1c6b064. Read the comment docs.