fix: Fix console errors about feature flags when running tests

[codyml]

SUMMARY

#21201 added a console error if feature flags are read before initialization, but the Jest test environment doesn't include a feature flag initialization step, so all tests that render components that check feature flags were printing lots of these console errors (though still passing). This PR sets featureFlags to an empty object in the global window shim object used by Jest.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

• Run Jest tests and search the output for any console errors with "feature flag". After this update, there should be none.

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 #21275 (0f28ae8) into master (9c4ae2a) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #21275      +/-   ##
==========================================
- Coverage   66.44%   66.44%   -0.01%     
==========================================
  Files        1784     1784              
  Lines       68234    68236       +2     
  Branches     7265     7265              
==========================================
- Hits        45341    45336       -5     
- Misses      21024    21030       +6     
- Partials     1869     1870       +1     

Flag	Coverage Δ
javascript	52.48% <ø> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
superset-frontend/src/featureFlags.ts	28.57% <0.00%> (-71.43%)	⬇️
superset-frontend/src/SqlLab/actions/sqlLab.js	61.05% <0.00%> (-0.33%)	⬇️
...d/src/SqlLab/components/SaveDatasetModal/index.tsx	52.87% <0.00%> (ø)

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[geido]

LGTM!

[michael-s-molina]

Thanks for the fix @codyml!