[JENKINS-71406] Flake in NodeCanTakeTaskTest.becauseNodeIsBusy

[jglick]

See JENKINS-71406. Similar to #8068, adapting to jenkinsci/jenkins-test-harness#198, though a more straightforward fix in this case: the test was indeed scheduling two builds and aborting and waiting for the first to complete, but neglected to do the same for the second before exiting.

Testing done

Just checked that it passed locally; did not observe a local failure. Test output with fix does show it aborting build 2, matching flake error message. Saw a flake in trunk recently: https://github.com/jenkinsci/jenkins/runs/14057889710

Proposed changelog entries

• N/A

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