KMS tests extremely slow in Mac OS build

e.g (from https://github.com/aws/aws-encryption-sdk-c/pull/574/checks?check_run_id=977613014)

```
Start 2: t_kms_keyring.test
2/22 Test #2: t_kms_keyring.test ............... Passed 484.45 sec
```

That test takes less than a second on my laptop. Wat.

(Was "KMS tests are spuriously passing in Mac OS build" - this was is a false alarm: `t_kms_keyring.test` mocks out the KMS client and does not need AWS credentials. I was misled by the superficially realistic-looking CMK ARN in the test code. :) Original description below:

The GitHub Actions-based build (.github/workflows/osx.yml) does not provide any AWS credentials, so it should not be possible for any tests that depend on KMS to work.

They are likely passing because the tests are accidentally ignoring error return values at some point.

Start with .github/workflows/osx.yml and the t_kms_keyring.test target, then compare the macOS test invocation with the local run described in the issue. Determine why the test takes about 484 seconds in the build and confirm that the relevant KMS test completes promptly without requiring AWS credentials.

Written by the indexing model from the issue text.