Ensure Tests Run Independently of Environment Variables

[petersutter]

What would you like to be added:
The test suite should be updated to ensure that tests run independently of the environment, regardless of any pre-existing environment variables that may be set locally.

Why is this needed:
When running the tests locally, they may fail if certain environment variables are set (E.g. when FGA_API_URL is set using PR #1960). This creates inconsistencies and makes it difficult for developers to reliably execute tests in their local environments.