DAOS-19031 cart: Fix test_ep_cred_client race condition

[frostedcmos]

• Fix test race condition where status was checked outside of the completion callback.

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'cart/rpc_two_node.py:CartRpcTwoNodeTest.test_cart_rpc_two_node - TestFail: Failed, return codes client 0 server -1'
Status is 'In Progress'
Labels: 'ci_master_daily'
https://daosio.atlassian.net/browse/DAOS-19031

[daosbuild3]

Test stage Functional Hardware Medium MD on SSD completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18439/1/testReport/

[phender]

Test stage Functional Hardware Medium MD on SSD completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18439/1/testReport/

Known issue: https://daosio.atlassian.net/browse/DAOS-19005