Issue #25122: sync test_eintr with Python 3.6

* test_eintr: support verbose mode, don't redirect eintr_tester output into
  a pipe
* eintr_tester: replace os.fork() with subprocess to have a cleaner child
  process (ex: don't inherit setitimer())
* eintr_tester: kill the process if the unit test fails
* test_open/test_os_open(): write support.PIPE_MAX_SIZE bytes instead of
  support.PIPE_MAX_SIZE*3 bytes