tests: prevent unexpected message combining

If the qrexec daemon is slow enough, it is possible for the standard
output of the service to be sent as one message instead of two.  Prevent
this by waiting for the first message to be received before sending
the service's standard input.  This also requires that the test service
be changed to read its input after producing the output from the
wait-for-session script.

Author: DemiMarie

qrexec/tests/socket/daemon.py

@@ -759,10 +759,10 @@ def test_run_dom0_service_wait_for_session(self):
             "rpc",
             "qubes.Service",
             """\
-        #!/bin/sh
-        read input
-        cat {}
-        echo "arg: $1, remote domain: $QREXEC_REMOTE_DOMAIN, input: $input"
+#!/bin/sh
+cat {}
+read input
+echo "arg: $1, remote domain: $QREXEC_REMOTE_DOMAIN, input: $input"
 """.format(
                 log
             ),
@@ -774,16 +774,16 @@ def test_run_dom0_service_wait_for_session(self):
 
         cmd = "QUBESRPC qubes.Service+arg src_domain name src_domain"
         source = self.connect_service_request(cmd)
+        self.assertEqual(source.recv_message(), (
+            qrexec.MSG_DATA_STDOUT,
+            b"wait for session: remote domain: src_domain\n",
+        ))
 
         source.send_message(qrexec.MSG_DATA_STDIN, b"stdin data\n")
         source.send_message(qrexec.MSG_DATA_STDIN, b"")
         self.assertEqual(
             source.recv_all_messages(),
             [
-                (
-                    qrexec.MSG_DATA_STDOUT,
-                    b"wait for session: remote domain: src_domain\n",
-                ),
                 (
                     qrexec.MSG_DATA_STDOUT,
                     b"arg: arg, remote domain: src_domain, "