Prior to this commit, we had a single test for `FeatureFlag.enabled?`,
namely that it delegates to `Flipper.enabled?`. This commit adds some
tests to both verify behavior and communicate to developers the
intentions of the related methods:
- `FeatureFlag.enabled?`
- `FeatureFlag.accessible?`
My hope is the documentation and tests will help address the horrors
mentioned in conversation in #16416:
> I have lived the horrors of having a feature flag accidently
> flipped or incorrectly setup in the first place and half baked
> functionality going live.
Note: I'm not proposing that we test other `Flipper` methods, but
instead to ensure that we're testing and documenting the behavior of
what I believe to be the two primary mechanisms of "putting something
behind a feature flag."
Related to #16406, #16416
