test: Fix async tests in SaveDatasetModal and complete overwrite screen test

[lyndsiWilliams]

SUMMARY

This PR fixes the final test in SaveDatasetModal.test.tsx. Initially the test for the overwrite screen buttons had to be squashed into the previous test because creating another test after this test would always render incorrectly and fail. The problem was that timers used by the component were being fired after the test ran. The solution was to use jest.useFakeTimers() at the top of the file, so that the timers run in the test environment. I also completed testing on the overwrite screen and cleaned up some arbitrary act wrappers.

TESTING INSTRUCTIONS

• In your terminal, cd into superset-frontend
• Run the command npm run test superset-frontend/src/SqlLab/components/SaveDatasetModal/SaveDatasetModal.test.tsx
• Observe that all tests pass with no act errors

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 #21020 (e543449) into master (e350823) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #21020      +/-   ##
==========================================
- Coverage   66.34%   66.28%   -0.07%     
==========================================
  Files        1767     1767              
  Lines       67281    68032     +751     
  Branches     7139     7398     +259     
==========================================
+ Hits        44639    45093     +454     
- Misses      20815    21063     +248     
- Partials     1827     1876      +49     

Flag	Coverage Δ
hive	53.14% <ø> (ø)
javascript	52.17% <ø> (+0.06%)	⬆️
mysql	80.92% <ø> (ø)
postgres	80.99% <ø> (ø)
presto	53.04% <ø> (ø)
python	81.47% <ø> (ø)
sqlite	79.58% <ø> (ø)
unit	50.47% <ø> (ø)

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

Impacted Files	Coverage Δ
...frontend/src/explore/actions/datasourcesActions.ts	100.00% <0.00%> (ø)
.../src/views/CRUD/data/dataset/DatasetPage/index.tsx	0.00% <0.00%> (ø)
...set-ui-core/src/ui-overrides/ExtensionsRegistry.ts	100.00% <0.00%> (ø)
...c/views/CRUD/data/database/DatabaseModal/styles.ts	76.78% <0.00%> (+0.31%)	⬆️
...t-frontend/src/explore/actions/saveModalActions.js	98.75% <0.00%> (+0.41%)	⬆️
...set-frontend/src/explore/actions/exploreActions.ts	59.52% <0.00%> (+1.19%)	⬆️
...s/CRUD/data/database/DatabaseModal/ModalHeader.tsx	68.57% <0.00%> (+1.90%)	⬆️
...et-frontend/src/explore/reducers/exploreReducer.js	43.00% <0.00%> (+3.00%)	⬆️
.../explore/components/ExploreViewContainer/index.jsx	55.36% <0.00%> (+3.16%)	⬆️
...c/views/CRUD/data/database/DatabaseModal/index.tsx	34.94% <0.00%> (+3.19%)	⬆️
... and 5 more

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