test: fix flaky test-readline-interface

Move test reliant on timer triggering in a timely fahion from parallel
to sequential. The test can fail under high load when the timer is
triggered too late and the `\r` and `\n` are treated as separate lines.

PR-URL: https://github.com/nodejs/node/pull/15066


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>