test: verify cluster worker exit

[cjihrig]

Checklist

• tests and code linting passes
• a test and/or benchmark is included
• the commit message follows commit guidelines

Affected core subsystem(s)

test

Description of change

test-cluster-disconnect-handles already includes logic that tries to cleanup any child processes when the test fails. This commit adds additional checks to verify that the child exited normally, and fails the test if that is not the case.

Refs: #6988