test: attempt to reduce inter-testcase flakiness in CI
Ex:
```
=== ERROR! init_per_testcase crashed!
Location: [{emqx_auth_mongo_SUITE,'-init_mongo_data/0-fun-0-',207},
{emqx_auth_mongo_SUITE,init_mongo_data,207},
{emqx_auth_mongo_SUITE,init_per_testcase,177},
{test_server,do_init_per_testcase,1554},
{test_server,run_test_case_eval1,1255},
{test_server,run_test_case_eval,1225}]
Reason: {{assertMatch,[{module,emqx_auth_mongo_SUITE},
{line,207},
{expression,"mongo_api : insert ( Connection , ? MONGO_CL_USER , ? INIT_AUTH )"},
{pattern,"{ { true , _ } , _ }"},
{value,{error,timeout}}]},
[{emqx_auth_mongo_SUITE,'-init_mongo_data/0-fun-0-',1,
[{file,"/emqx/apps/emqx_auth_mongo/test/emqx_auth_mongo_SUITE.erl"},
{line,207}]},
{emqx_auth_mongo_SUITE,init_mongo_data,0,
[{file,"/emqx/apps/emqx_auth_mongo/test/emqx_auth_mongo_SUITE.erl"},
{line,207}]},
{emqx_auth_mongo_SUITE,init_per_testcase,2,
[{file,"/emqx/apps/emqx_auth_mongo/test/emqx_auth_mongo_SUITE.erl"},
{line,177}]},
{test_server,do_init_per_testcase,2,[{file,"test_server.erl"},{line,1554}]},
{test_server,run_test_case_eval1,6,[{file,"test_server.erl"},{line,1255}]},
{test_server,run_test_case_eval,9,[{file,"test_server.erl"},{line,1225}]}]}
```
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