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

Lib/test/test_eintr.py

 import os
 import signal
+import subprocess
+import sys
 import unittest
 
 from test import support
@@ -14,7 +16,14 @@ class EINTRTests(unittest.TestCase):
         # Run the tester in a sub-process, to make sure there is only one
         # thread (for reliable signal delivery).
         tester = support.findfile("eintr_tester.py", subdir="eintrdata")
-        script_helper.assert_python_ok(tester)
+
+        if support.verbose:
+            args = [sys.executable, tester]
+            with subprocess.Popen(args) as proc:
+                exitcode = proc.wait()
+            self.assertEqual(exitcode, 0)
+        else:
+            script_helper.assert_python_ok(tester)
 
 
 if __name__ == "__main__":