test: mark test-fs-readfile-tostring-fail as flaky

test-fs-readfile-tostring-fail is failing frequently on OSX machines.
There's a PR to fix this issue in libuv, but while the fix don't land on
Node.js this test should be marked as flaky.

Ref: https://github.com/nodejs/node/issues/16601
Ref: https://github.com/libuv/libuv/pull/1742

PR-URL: https://github.com/nodejs/node/pull/21013
Refs: https://github.com/nodejs/node/issues/16601
Refs: https://github.com/libuv/libuv/pull/1742


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>