Add stable concurrent option to react-test-renderer (#27804)

## Summary

Concurrent rendering has been the default since React 18 release.
ReactTestRenderer requires passing `{unstable_isConcurrent: true}` to
match this behavior, which means by default tests written with RTR use a
different rendering method than the code they test.

Eventually, RTR should only use ConcurrentRoot. As a first step, let's
add a version of the concurrent option that isn't marked unstable. Next
we will follow up with removing the unstable option when it is safe to
merge.

## How did you test this change?

`yarn test
packages/react-test-renderer/src/__tests__/ReactTestRendererAsync-test.js`