feature/add-first-class-debugging-support-for-tests

Created by: cdaringe

problem statement

• react-scripts does not offer ready-to-roll jest debugging.

solution

• add it!
  • i selected a natural model where --debug-brk and debug are working flags passed to npm test. that is npm test -- --debug-brk works. i think this is a nice entry point into the system.
  • auto-handle other flags that need to be massaged to make debugging work. that is, disable watch and enable runInBand

closes #594 (closed)