Do not close stdin, stdout, or stderr

Instead, point them at /dev/null.  Closing standard file descriptors is
almost always a bug, and in this case causes a hang in the tests.

qrexec/tests/socket/agent.py

@@ -938,10 +938,10 @@ def test_service_close_stdout_stderr_early(self):
 #!/bin/sh
 read
 echo closing stdout
-exec >&-
+exec >/dev/null
 read
 echo closing stderr >&2
-exec 2>&-
+exec 2>/dev/null
 read code
 exit $code
 """,
@@ -1028,7 +1028,7 @@ def test_close_stdin_early(self):
         target, dom0 = self.execute(
             """
 read
-exec <&-
+exec </dev/null
 echo "closed stdin"
 sleep 1
 """