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/

test/parallel/test-runner-exit-code.js

@@ -20,7 +20,7 @@ async function runAndKill(file) {
   });
   const [code, signal] = await once(child, 'exit');
   await finished(child.stdout);
-  assert.strictEqual(stdout, 'TAP version 13\n');
+  assert(stdout.startsWith('TAP version 13\n'));
   assert.strictEqual(signal, null);
   assert.strictEqual(code, 1);
 }