test(banned): attempt to fix flaky test
Example failure:
https://github.com/emqx/emqx/actions/runs/4821105856/jobs/8587006829#step:8:4495
```
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emqx_common_test_helpers:wait_for_down failed on line 434
Reason: {{t_session_taken,178,timeout},[{emqx_common_test_helpers,...},{...}|...]}
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Testing lib.emqx.emqx_banned_SUITE: *** FAILED test case 4 of 5 ***
%%% emqx_banned_SUITE ==> t_session_taken: FAILED
%%% emqx_banned_SUITE ==> {{t_session_taken,178,timeout},
[{emqx_common_test_helpers,wait_for_down,6,
[{file,"/__w/emqx/emqx/source/apps/emqx/test/emqx_common_test_helpers.erl"},
{line,434}]},
{emqx_banned_SUITE,t_session_taken,1,
[{file,"/__w/emqx/emqx/source/apps/emqx/test/emqx_banned_SUITE.erl"},
{line,176}]},
{test_server,ts_tc,3,[{file,"test_server.erl"},{line,1782}]},
{test_server,run_test_case_eval1,6,[{file,"test_server.erl"},{line,1291}]},
{test_server,run_test_case_eval,9,[{file,"test_server.erl"},{line,1223}]}]}
```
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