Back-patch test cases for timetz_zone/timetz_izone.

Back-patch test cases for timetz_zone/timetz_izone.

Per code coverage reports, we had zero regression test coverage
of these functions.  That came back to bite us, as apparently
that's allowed us to miss discovering misbehavior of this code
with AIX's xlc compiler.  Install relevant portions of the
test cases added in 97957fdba, 2f0472030, 19fa97731.

(Assuming the expected outcome that the xlc problem does appear
in back branches, a code fix will follow.)

Discussion: https://postgr.es/m/CA+hUKGK=DOC+hE-62FKfZy=Ybt5uLkrg3zCZD-jFykM-iPn8yw@mail.gmail.com