Minor improvements in file cache

[foodprocessor]

Describe your changes in brief

In file cache test suite:

• Re-enable tests that were flaky due to race conditions that are fixed now
• Shorten a couple test sleep times
• Remove unnecessary OpenFile when timeout is non-zero
• Remove redundant call to setupTestHelper with config identical to test default config

In lru_policy:

• Remove unused const
• Simplify cacheTimeoutMonitor code
  • (calling time.Tick with a zero interval already returns nil, so no need to check if the interval is zero)
• Add comments to updateMarker to explain linked list code

What type of Pull Request is this? (check all applicable)

• [ ] Refactor
• [ ] Feature
• [ ] Bug Fix
• [ ] Optimization
• [ ] Documentation Update

Checklist

• [ ] Tested locally
• [ ] Added new dependencies
• [ ] Updated documentation
• [ ] Added tests

Related Issues

• Related Issue #
• Closes #