Replace metro-memory-fs dependency in tests with OSS memfs (#52572)

Summary:

We'd like to stop supporting `metro-memory-fs` - there are better alternatives out there.

This replaces the one usage of `metro-memory-fs` in `react-native` with `memfs`, and swaps the dependency.

Changelog: [Internal]

Reviewed By: vzaidman

Differential Revision: D78161921