Allow statistics test rerun

This test has been unstable for a long while, especially the check
"statistics:verifying active sockets output in named.stats". Allow the
statistics test to be re-run to avoid frequent false positives.

bin/tests/system/statistics/tests_sh_statistics.py

@@ -9,6 +9,9 @@
 # See the COPYRIGHT file distributed with this work for additional
 # information regarding copyright ownership.
 
+import pytest_custom_markers
 
+
+@pytest_custom_markers.flaky(max_runs=2)  # GL#1621
 def test_statistics(run_tests_sh):
     run_tests_sh()