* Convert ReactSuspensePlaceholder tests to use noop
Instead of the test renderer, since test renderer does not support
running in persistent mode.
* Run Placeholder tests in persistent mode, too
* Fix Flow and lint
* Hidden text instances should have correct host context
Adds a test for a subtle edge case that only occurs in persistent mode.
* createHiddenTextInstance -> cloneHiddenTextInstance
This sidesteps the problem where createHiddenTextInstance needs access
to the host context.