https://github.com/facebook/react/pull/32529 added a dynamic flag for
this, but that breaks tests since the flags are not defined everywhere.
However, this is a static value and the flag is only for supporting
existing tests. So we can override it in the test config, and make it
static at built time instead.
## Overview
We didn't have any tests that ran in persistent mode with the xplat
feature flags (for either variant).
As a result, invalid test gating like in
https://github.com/facebook/react/pull/29664 were not caught.
This PR adds test flavors for `ReactFeatureFlag-native-fb.js` in both
variants.
