`ReactDebugTool` used to only call `purgeUnmountedComponents()` while profiling, so information about unmounted instances kept accumulating when not profiling.
Additionally, unmounting in React Native and rendering to string did not correctly clean up the devtool.
Finally, the tests tested the wrong behavior and relied on explicit `purgeUnmountedComponent()` calls.
To fix this, we:
* Test specifically that unmounting is enough to clean up the tree devtool.
* Add missing `onBeginFlush` and `onEndFlush` calls to server and native rendering so `ReactDebugTool` knows when to copy the tree.
Fixes#6750
3779a5d18c