support/testing: netcat: improve test robustness

Under some conditions (mostly slow execution due to test host load),
the netcat runtime test can randomly fail. This is due to several
facts:

- the sleep time between the server and client is too short,
- the use of netcat option -c could close the connection before the
  server could receive all the data.

This commit improves the test robustness by increasing the sleep time,
and by reducing the amount of transferred data (from 1MB of random data,
to a simpler string of few bytes). Also, to make sure netcat cannot wait
on DNS resolution, this commit also adds the -n option.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/6093854664

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>

support/testing/tests/package/test_netcat.py

@@ -22,20 +22,19 @@ class TestNetCat(infra.basetest.BRTest):
 
         self.assertRunOk("nc --version")
 
-        in_file = "input.bin"
-        out_file = "output.bin"
+        msg = "Hello Buildroot!"
+        out_file = "output.txt"
         port = 12345
 
-        cmd = f"dd if=/dev/urandom of={in_file} bs=1k count=1k"
+        cmd = f"nc -n -l -p {port} > {out_file} 2> /dev/null &"
         self.assertRunOk(cmd)
 
-        cmd = f"nc -l -p {port} > {out_file} &"
+        time.sleep(5)
+
+        cmd = f"echo '{msg}' | nc -n -c 127.0.0.1 {port}"
         self.assertRunOk(cmd)
 
-        time.sleep(1)
-
-        cmd = f"cat {in_file} | nc -c 127.0.0.1 {port}"
-        self.assertRunOk(cmd)
-
-        cmd = f"cmp {in_file} {out_file}"
-        self.assertRunOk(cmd)
+        cmd = f"cat {out_file}"
+        out, ret = self.emulator.run(cmd)
+        self.assertEqual(ret, 0)
+        self.assertEqual(out[0], msg)