dummyssh: make sure we don't inherit files descriptor to the children

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/D11225

tests/dummyssh

@@ -31,5 +31,5 b" if os.name == 'nt':"
     hgcmd = shlex.join(cmds)
     # shlex generate windows incompatible string...
     hgcmd = hgcmd.replace("'", '"')
-r = subprocess.call(hgcmd, shell=True)
+r = subprocess.call(hgcmd, shell=True, close_fds=True)
 sys.exit(bool(r))