support/testing: improve mosquitto test reliability

The mosquitto runtime test can randomly fail on slow
runners, see [1].

This commit improves this test in the following ways:

- the mosquitto_sub subscriber process is now started in a subshell
to suppress the job control messages (to prevent any spurious
messages when the job stops),

- the standard error is redirected to /dev/null, to prevent the
printing of any messages,

- the mosquitto_pub publisher process is started later, by increasing
the sleep time,

- finally, a new sleep time is introduced between the mosquitto_pub
publisher process and the check of the mosquitto_sub subscriber, to
make sure it will have time to write its output and exit.

Fixes: [1]

[1] https://gitlab.com/buildroot.org/buildroot/-/jobs/8453386454

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/testing/tests/package/test_mosquitto.py

@@ -25,13 +25,19 @@ class TestMosquitto(infra.basetest.BRTest):
         msg = "Hello Buildroot!"
 
         # We subscribe to a topic and write one message to a log file.
-        self.assertRunOk(f"mosquitto_sub -t {topic} -C 1 > {log} &")
+        cmd = f"( mosquitto_sub -t {topic} -C 1 > {log} 2> /dev/null & )"
+        self.assertRunOk(cmd)
 
-        time.sleep(1)
+        # We give some time to the subscriber process to settle.
+        time.sleep(5)
 
         # We publish a message.
         self.assertRunOk(f"mosquitto_pub -t {topic} -m '{msg}'")
 
+        # We give some more time to the subscriber process to write
+        # the log and quit.
+        time.sleep(5)
+
         # We check the log file contains our message.
         out, ret = self.emulator.run(f"cat {log}")
         self.assertEqual(ret, 0)