pgsql: Add test for CREATE INDEX CONCURRENTLY with not-so-immutable pre

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

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ce8949c4b6e225a3ae31e4b7ccdef0f7ab70ada6

Modified Files
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src/test/regress/expected/create_index.out | 12 ++++++++++++
src/test/regress/sql/create_index.sql      | 13 ++++++++++++-
2 files changed, 24 insertions(+), 1 deletion(-)