[Temporal] Reorganize tests for link time zones

Remove the staging test staging/sm/Temporal/ZonedDateTime/zones-and-links.js
and move its functionality into two files:
* test/intl402/Temporal/ZonedDateTime/links.js, which now combines most of the
functionality from zones-and-links.js with the functionality from
links-*.js in the same directory
* test/intl402/Temporal/ZonedDateTime/prototype/toString/timezone.js, which
factors out the testing for toString from zones-and-links.js.

Author: catamorphism