test: mark test-crypto-timing-safe-equal-benchmarks flaky

PR-URL: https://github.com/nodejs/node/pull/40065
Refs: https://github.com/nodejs/node/issues/38226


Reviewed-By: Ash Cripps <acripps@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>