Prepare placeholders before timing out (#13092)

* Prepare placeholders before timing out

While a tree is suspended, prepare for the timeout by pre-rendering the
placeholder state.

This simplifies the implementation a bit because every render now
results in a completed tree.

* Suspend inside an already timed out Placeholder

A component should be able to suspend inside an already timed out
placeholder. The time at which the placeholder committed is used as 
the start time for a subsequent suspend.

So, if a placeholder times out after 3 seconds, and an inner
placeholder has a threshold of 2 seconds, the inner placeholder will
not time out until 5 seconds total have elapsed.

Author: acdlite

src/ReactTestHostConfig.js

@@ -32,6 +32,8 @@ export type PublicInstance = Instance | TextInstance;
 export type HostContext = Object;
 export type UpdatePayload = Object;
 export type ChildSet = void; // Unused
+export type TimeoutHandle = TimeoutID;
+export type NoTimeout = -1;
 
 export * from 'shared/HostConfigWithNoPersistence';
 export * from 'shared/HostConfigWithNoHydration';
@@ -187,6 +189,10 @@ export const scheduleDeferredCallback =
 export const cancelDeferredCallback =
   TestRendererScheduling.cancelDeferredCallback;
 
+export const scheduleTimeout = setTimeout;
+export const cancelTimeout = clearTimeout;
+export const noTimeout = -1;
+
 // -------------------
 //     Mutation
 // -------------------