[9.18] chg: ci: Allow rerun of unstable statschannel tests under TSAN

The test_traffic_json and test_traffic_xml occasionally fail when
running under TSAN. This happens in CI and is most likely a result of
some instability that doesn't seem to be easily reproduced.

Closes #4598

Backport of MR !9293

Merge branch 'backport-4598-mark-statschannel-test-flaky-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9351