Issue #25654:

* multiprocessing: open file with closefd=False to avoid ResourceWarning
* _test_multiprocessing: open file with O_EXCL to detect bugs in tests (if a
  previous test forgot to remove TESTFN)
* test_sys_exit(): remove TESTFN after each loop iteration

Initial patch written by Serhiy Storchaka.
diff --git a/Lib/multiprocessing/process.py b/Lib/multiprocessing/process.py
index 68959bf..bca8b7a 100644
--- a/Lib/multiprocessing/process.py
+++ b/Lib/multiprocessing/process.py
@@ -234,12 +234,7 @@
                 context._force_start_method(self._start_method)
             _process_counter = itertools.count(1)
             _children = set()
-            if sys.stdin is not None:
-                try:
-                    sys.stdin.close()
-                    sys.stdin = open(os.devnull)
-                except (OSError, ValueError):
-                    pass
+            util._close_stdin()
             old_process = _current_process
             _current_process = self
             try: