run-tests: do not inherit file descriptor when running a command

This is one of the difference between python2 and python3 and could have been a
reason why test hang with python2 + chg. This does not seems to help the
hanging issue at all…

However, now that this is written lets reduce the difference between python2
and python3.

Differential Revision: https://phab.mercurial-scm.org/D11226

tests/run-tests.py

@@ -1587,6 +1587,7 b' class Test(unittest.TestCase):'
             proc = subprocess.Popen(
                 _bytes2sys(cmd),
                 shell=True,
+                close_fds=closefds,
                 cwd=_bytes2sys(self._testtmp),
                 env=env,
             )