Allow nsupdate test rerun on FreeBSD

The "exceeded time limit waiting for literal 'too many DNS UPDATEs
queued' in ns1/named.run" is prone to fail due to a timing issue.
Despite out efforts to stabilize it, the check still often fails on
FreeBSD in our CI. Allow the test to be re-run on this platform.

(cherry picked from commit 124882476b)

bin/tests/system/nsupdate/tests_sh_nsupdate.py

@@ -9,6 +9,14 @@
 # See the COPYRIGHT file distributed with this work for additional
 # information regarding copyright ownership.
 
+import platform
 
+import pytest_custom_markers
+
+
+MAX_RUNS = 2 if platform.system() == "FreeBSD" else 1  # GL#3846
+
+
+@pytest_custom_markers.flaky(max_runs=MAX_RUNS)
 def test_nsupdate(run_tests_sh):
     run_tests_sh()