LibJS: Now.plainDateTimeISO.js custom time zone test is very flaky

[trflynn89]

These have been flaky for quite a while, with the following error:

   FAIL  builtins/Temporal/Now/Now.plainDateTime.js
       ❌ Suite:  correct behavior
           Test:   custom time zone positive (failed):
                   ExpectationError: toBe: expected _86400_, got _86399_

   FAIL  builtins/Temporal/Now/Now.plainDateTimeISO.js
       ❌ Suite:  correct behavior
           Test:   custom time zone positive (failed):
                   ExpectationError: toBe: expected _86400_, got _86399_

Quite a bit of work has gone into making these less flaky in the past, see:
c0a3b14
1b0ca52
cf0d30a

However, they still flake, and should be investigated (or maybe it becomes a non-issue when we catch up to the latest Temporal spec).