Deflake caching test 'Can load cached indexed data'

[Ethan-Arrowood]

The test file unitTests/resources/caching.test.js has been flaky on harper CI, I’ve noticed it in multiple PR CI runs recently.

• Latest example: Improve integration test harperLifecycle: rename setupHarper, add docs for killHarper/logDir/env vars #235 (Only modifies Integration Tests)
• Test “Can load cached indexed data”
  • On Node v22, error occurred on line 325: “TypeError: result.getExpiresAt is not a function”
  • On Node v20, error occurred on line 316: “AssertionError 3 == 2”

Can reproduce locally using:

for i in $(seq 1 50); do npm run test:unit -- unitTests/resources/caching.test.js --grep 'Can load cached indexed data'; done

The “TypeError: result.getExpiresAt is not a function” is easy to solve as we can just remove the last two lines that have no impact on the actual test:

		result = await IndexedCachingTable.get(23);
		console.log(result.getExpiresAt());

But the other error is more difficult. Bumping the timeout to 20ms didn't work, and mocha explicitly recommends avoiding using 1this.retries()` for unit tests. I'm really looking for a more deterministic solution here, or potentially a deep dive into why this specific circumstance is flaky when other tests in this file are not. This test file makes have use of timers so why is only this test flaking?

[Ethan-Arrowood]

Related to #239