test: mark test-runner-output as flaky

This has been flaky on many platforms for months. Mark it as
flaky for now to avoid blocking the CI.

PR-URL: #49854
Refs: #49853
Refs: nodejs/reliability#673
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: LiviaMedeiros <livia@cirno.name>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/parallel.status

@@ -5,6 +5,8 @@ prefix parallel
 # sample-test                        : PASS,FLAKY
 
 [true] # This section applies to all platforms
+# https://github.com/nodejs/node/issues/49853
+test-runner-output: PASS,FLAKY
 
 [$system==win32]
 # https://github.com/nodejs/node/issues/41206