support/testing: TestMdnsd: improve test reliability

The mdnsd runtime test can randomly fail on slow runners.

It's hard to reproduce locally (only one failure after a few attempts)
but we can reproduce it easily by removing the while loop entirely.

It turns out that mdnsd is started by S50mdnsd before the
emulator.login() change the system date:

  [BRTEST# date -s @1788032864
  Sat Aug 29 19:47:44 UTC 2026

Since the minimal rootfs.cpio generated	for TestMdnsd doesn't have any
ntp client installed, it start with "January 1, 1970".

The date change may cause some issue to the mdnsd daemon which blocks
any response from mquery command.

When the problem occurs, "mquery -T _http._tcp" reply is empty:

  # mquery -T _http._tcp
  Querying _http._tcp.local. for PTR (12) ... press Ctrl-C to stop

To workaround the issue, restart mdnsd manually.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/16185948555

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

support/testing/tests/package/test_mdnsd.py

@@ -23,6 +23,10 @@ class TestMdnsd(infra.basetest.BRTest):
                                     "-net", "user"])
         self.emulator.login()
 
+        # Restart mdnsd after setting the date in emulator.login() setup.
+        cmd = "/etc/init.d/S50mdnsd restart"
+        self.assertRunOk(cmd, timeout=30)
+
         # We check the program can execute.
         self.assertRunOk("mdnsd -v")