Add regression test coverage for timetz_izone().
Extend the test added by commit
97957fdba so that it also covers
timetz_izone(), that is the "AT TIME ZONE interval" case.
This is mostly to see if xlc's apparent bug occurs there too,
but more code coverage is always welcome.
Discussion: https://postgr.es/m/
2287835.
1697464481@sss.pgh.pa.us