DAOS-18086 control: Fix data race in drpc unit test

[kjacque]

This issue was only affecting unit tests.

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).

[kjacque]

Scripted "go test -race" runs for this test 100 times in a row. I was able to reproduce the original issue within 100 runs on master, but not with this patch.

[github-actions]

Ticket title is 'Unit Tests / Unit Test on EL 8.8 / UTEST_control.drpc.TestServer_Listen_AcceptConnection: data race'
Status is 'In Review'
https://daosio.atlassian.net/browse/DAOS-18086