Temporal.now missing tests for user-supplied time zones

[justingrant]

While building #1130 I noticed that there are no tests for the user-supplied time zone variations of Temporal.now. All the existing tests (except for the ones I added for ZDT) only use the default system time zone.

[ptomato]

This is a polyfill or test262 thing and isn't a requirement for Stage 3.

[ptomato]

We now have test262 tests for user-provided time zones in Temporal.Now.