script_helper: kill the subprocess on error

If Popen.communicate() raises an exception, kill the child process to not leave
a running child process in background and maybe create a zombi process.

This change fixes a ResourceWarning in Python 3.6 when unit tests are
interrupted by CTRL+c.

Lib/test/support/script_helper.py

@@ -83,16 +83,16 @@ def run_python_until_end(*args, **env_vars):
         env = {}
     env.update(env_vars)
     cmd_line.extend(args)
-    p = subprocess.Popen(cmd_line, stdin=subprocess.PIPE,
+    proc = subprocess.Popen(cmd_line, stdin=subprocess.PIPE,
                          stdout=subprocess.PIPE, stderr=subprocess.PIPE,
                          env=env)
-    try:
-        out, err = p.communicate()
-    finally:
-        subprocess._cleanup()
-        p.stdout.close()
-        p.stderr.close()
-    rc = p.returncode
+    with proc:
+        try:
+            out, err = proc.communicate()
+        finally:
+            proc.kill()
+            subprocess._cleanup()
+    rc = proc.returncode
     err = strip_python_stderr(err)
     return _PythonRunResult(rc, out, err), cmd_line