dillon1000/react

Wrap fiber-only test in feature flag (#9665)

**what is the change?:**
A test was added for a change to Fiber's behavior in #9608, and because of a
bug in our CirclCI script it landed when failing for non-fiber runs of the
tests.

This just wraps the test in a feature flag because it seems clear it was
only intended to test the new fiber behavior.

Thanks to @gaearon for pairing on this! :)

**why make this change?:**
So that tests are passing on master.

**test plan:**
`npm run test ReactCompositeComponentState`

**issue:**
None - figured it out before anyone opened an issue afaik.