[FEATURE] Restore integration test stability

[skyerus]

Requirements

Changes to flagd's sync mechanisms have caused the integration tests to become unstable. Investigate & eliminate points of instability.

[skyerus]

The problem is that the tests replace the contents of the flag configuration file with the amended configuration (to test that cache has been invalidated). In doing this, flagd receives the change event, pulls the contents of the file, sees that the file is empty (it hasn't been written to yet), interprets this as an empty state and invalidates all of the cache. Then the configuration gets written to the file and flagd interprets the event correctly.

Potential solutions:

• Write config to new file and create symlink from existing file
• Rerun tests on failure (slow)
• Retry retrieval if file is empty
• Run the tests against the flagd binary directly (it seems that the slowness of the write/retrieval comes from the mounting of the file to the docker volume) Tried this, the failure occurs in the CI regardless: https://github.com/open-feature/flagd/actions/runs/4244119058/jobs/7377781110