fix(tests): prevent SemanticLayerModal test from hanging indefinitely

[aminghadersohi]

Summary

• The SemanticLayerModal test introduced in feat: semantic layer extension #37815 hangs forever, causing shard 6 of the frontend test suite to time out after 3+ hours
• The test uses fake timers to control a debounce, but our test environment patches out MessageChannel (to work around an rc-overflow issue). Without MessageChannel, React falls back to setTimeout for scheduling renders — and fake timers block those too, so React never re-renders and the test never finishes
• Fix: tell Jest to auto-advance fake timers (advanceTimers: true), which unblocks React's render scheduling while still letting the test explicitly control the 500ms debounce
• Also reorder the DatasetList.test.tsx cleanup to restore real timers before flushing async work, so a test failure mid-run can't cause the same kind of hang

Test plan

• SemanticLayerModal.test.tsx passes in ~14s (was hanging indefinitely)
• All 28 DatasetList.test.tsx tests pass
• Shard 6/8 completes in CI without timeout