test: optimize codecov config

[ktmud]

SUMMARY

Optimize CodeCov config to

1. Always let GitHub CodeCov checks pass because we don't always report all test coverage (because of conditional CI jobs)
2. Add more detailed flags for coverage uploads
3. Run tests also on workflow file changes

It seems the CodeCov App has recently been enabled in this repo (#9930), which is good, but it starts to block some PRs on status check due to false alarms of decreased test coverage.

This is because:

1. The target coverage setting is set to "auto", which will report any decrease in test coverage
2. We conditionally skip Python tests for pure frontend changes (and vice versa), hence certain PRs will never have the full test coverage reported.
3. Sometimes frontend and backend workflows are delayed in queue and run in very different time. After wait for a while, CodeCov may come to believe the full CI has finished, therefore reporting only partial coverage from the already finished jobs.

The after_n_builds config is designed to address this issue, but since we don't know which jobs will run for each PR, we can't really set a meaningful value for this config.

I'm setting it to 4 because for Python tests there are 5 uploads (mysql, postgres, sqlite, hive, presto), and for frontend, there are 4 (unit tests + Cypress * 3).

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TEST PLAN

• CI Passes and flags correctly marked on CodeCov
  
• GH checks are not blocked by CodeCov

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #13329 (c694ee9) into master (8ab45c9) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #13329      +/-   ##
==========================================
- Coverage   77.08%   77.01%   -0.07%     
==========================================
  Files         876      876              
  Lines       45439    45439              
  Branches     5440     5440              
==========================================
- Hits        35026    34997      -29     
- Misses      10290    10319      +29     
  Partials      123      123              

Flag	Coverage Δ
cypress	58.16% <ø> (ø)
hive	79.97% <ø> (?)
javascript	62.27% <ø> (ø)
mysql	80.30% <ø> (?)
postgres	80.34% <ø> (?)
python	80.73% <ø> (-0.12%)	⬇️
sqlite	79.96% <ø> (?)

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

Impacted Files	Coverage Δ
superset/db_engine_specs/presto.py	82.47% <0.00%> (-5.99%)	⬇️
superset/models/core.py	88.55% <0.00%> (-0.28%)	⬇️

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[villebro]

LGTM

[willbarrett]

Thank you Jesse for the optimization! Apologies for the unintended consequences.