Allow dnstap system test rerun on freebsd13

The check "unix socket message counts" sometimes fails with "dnstap
output file smaller than expected". This only happens on freebsd13 and
can't be reproduced easily. There was an attempt to decrease the
required file size in the past, but apparently, the issue can still
occur.

(cherry picked from commit 34867e1693)

bin/tests/system/dnstap/tests_sh_dnstap.py

@@ -26,5 +26,6 @@ pytestmark = pytest.mark.extra_artifacts(
 )
 
 
+@isctest.mark.flaky(max_runs=2, rerun_filter=isctest.mark.is_host_freebsd_13)
 def test_dnstap(run_tests_sh):
     run_tests_sh()

bin/tests/system/isctest/mark.py

@@ -13,6 +13,7 @@
 
 import os
 from pathlib import Path
+import platform
 import shutil
 import ssl
 import subprocess
@@ -54,6 +55,10 @@ def is_dnsrps_available():
     return True
 
 
+def is_host_freebsd_13(*_):
+    return platform.system() == "FreeBSD" and platform.release().startswith("13")
+
+
 have_libxml2 = pytest.mark.skipif(
     not feature_test("--have-libxml2"), reason="libxml2 support disabled in the build"
 )

bin/tests/system/timeouts/tests_tcp_timeouts.py

@@ -13,7 +13,6 @@
 
 # pylint: disable=unused-variable
 
-import platform
 import socket
 import time
 
@@ -68,11 +67,7 @@ def test_initial_timeout(named_port):
                 raise EOFError from e
 
 
-def is_host_freebsd_13(*_):
-    return platform.system() == "FreeBSD" and platform.release().startswith("13")
-
-
-@isctest.mark.flaky(max_runs=2, rerun_filter=is_host_freebsd_13)
+@isctest.mark.flaky(max_runs=2, rerun_filter=isctest.mark.is_host_freebsd_13)
 def test_idle_timeout(named_port):
     #
     # The idle timeout is 5 seconds, so the third message should fail