readline: use Date.now() and move test to parallel

The readline module wants a truthy time while using Timer.now() doesn't
necessarily guarantee that early on in the process' life. It also
doesn't actually resolve the timing issues experienced in an earlier
issue. Instead, this PR fixes the related tests and moves them back
to parallel.

Refs: https://github.com/nodejs/node/issues/14674

PR-URL: https://github.com/nodejs/node/pull/18563


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>