Minor backstage spec changes

3f661f5f - Set a short base interval for Retriable in specs

This prevents Retriable blocks from causing delays during testing.

9725f1df - Fix feature stubbing

Previously if we explicitly enabled one feature, all of the others would get disabled. This change first allows any non-specific Feature.enabled? call by delegating to the original implementation, and then stubbing a call with a specific argument to return the desired value.