test: fix flaky test-runner-exit-code.js
test/parallel/test-runner-exit-code.js failed in a recent CI run with the following error:
+ actual - expected
+ 'TAP version 13\n' +
+ '# Subtest: /home/iojs/build/workspace/node-test-commit-linuxone/test/fixtures/test-runner/never_ending_async.js\n'
- 'TAP version 13\n'
This commit resolves that flakiness.
Refs: https://ci.nodejs.org/job/node-test-commit-linuxone/35759/