On Windows, ICU does not look at the TZ environment variable at all. So
to support changing time zones in test-js, let's set ICU's default time
zone directly.
Note that we no longer deal with "null" time zone strings. We just cache
whatever ICU thinks is the current time zone before attempting to change
it, for which we never have a null result.
Co-authored-by: Andrew Kaster <andrew@ladybird.org>