Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48097
Changelog: [internal]
As per title, this allows us to specify both common and JS-only feature flags for tests in the docblock as pragmas (in the same pragma separated by spaces, or in different pragmas). E.g.:
```
/**
* fantom_flags commonTestFlag:true
* fantom_flags jsOnlyTestFlag:true
*/
```
The feature flags are overridden automatically for us before the tests start.
Reviewed By: javache
Differential Revision: D66760121
fbshipit-source-id: 7e227e0035a170dab81b1e6ce39600a01a748867