Test for duplicate key import

[TingDaoK]

• add a regression test for https://github.com/awslabs/aws-c-io/pull/626
  • We can see the CI failing for OSX https://github.com/awslabs/aws-c-io/actions/runs/7994446706/job/21832512891 without the fix
  • After the fix, the CI passed
• also re-enable the test for test_concurrent_cert_import, the previous issue seems to be fixed

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1779683) 80.04% compared to head (08cc9ce) 80.04%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #627 +/- ## ======================================= Coverage 80.04% 80.04% ======================================= Files 28 28 Lines 5949 5949 ======================================= Hits 4762 4762 Misses 1187 1187 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.