chore: add coverage command in package.json

[zhaoyongjie]

SUMMARY

The master branch is usually broken by coverage rate of superset-ui/core. this PR adds 1) a coverage cmd in package.json, 2) add coverageThreshold in this command, I will add a new step in the frontend unit test workflow in the separated PR.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

npm run core:cover

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #20769 (2d771b8) into master (e60083b) will increase coverage by 0.00%.
The diff coverage is 88.88%.

❗ Current head 2d771b8 differs from pull request most recent head b234920. Consider uploading reports for the commit b234920 to get more accurate results

@@           Coverage Diff           @@
##           master   #20769   +/-   ##
=======================================
  Coverage   66.35%   66.35%           
=======================================
  Files        1754     1754           
  Lines       66689    66693    +4     
  Branches     7049     7049           
=======================================
+ Hits        44253    44257    +4     
  Misses      20639    20639           
  Partials     1797     1797           

Flag	Coverage Δ
javascript	51.93% <88.88%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...d/src/SqlLab/components/SqlEditorLeftBar/index.tsx	49.20% <0.00%> (ø)
superset-frontend/src/SqlLab/actions/sqlLab.js	60.44% <100.00%> (+0.22%)	⬆️
...ntend/src/SqlLab/components/TableElement/index.tsx	72.30% <100.00%> (ø)
superset-frontend/src/SqlLab/reducers/sqlLab.js	33.33% <100.00%> (+0.71%)	⬆️

[stephenLYZ]

LGTM

[kgabryje]

LGTM