test: fix flakyness in test-runner reporter test

PR-URL: #45930
Fixes: #45929
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

test/parallel/test-runner-reporters.js

@@ -89,7 +89,9 @@ describe('node:test reporters', { concurrency: true }, () => {
                               ['--test', '--test-reporter', fixtures.path('test-runner/custom_reporters/', filename),
                                testFile]);
       assert.strictEqual(child.stderr.toString(), '');
-      assert.strictEqual(child.stdout.toString(), `${filename} {"test:start":5,"test:pass":2,"test:fail":3,"test:plan":3,"test:diagnostic":7}`);
+      const stdout = child.stdout.toString();
+      assert.match(stdout, /{"test:start":5,"test:pass":2,"test:fail":3,"test:plan":3,"test:diagnostic":\d+}$/);
+      assert.strictEqual(stdout.slice(0, filename.length + 2), `${filename} {`);
     });
   });
 });