Add test for CREATE INDEX CONCURRENTLY with not-so-immutable predicate
83158f7 has improved index_set_state_flags() so as it is possible to use
transactional updates when updating pg_index state flags, but there was
not really a test case which stressed directly the possibility it fixed.
This commit adds such a test, using a predicate that looks valid in
appearance but calls a stable function.
Author: Andrey Lepikhov
Discussion: https://postgr.es/m/
9b905019-5297-7372-0ad2-
e1a4bb66a719@postgrespro.ru
Backpatch-through: 9.6