Fix database creation during installchecks for ICU cluster

Hello!

This is a copy of [1] moved to a separated thread for Commitfest..

I discovered an interesting behaviour during installcheck runs on PG 15+ 
when the cluster was initialized with ICU locale provider:

$ initdb --locale-provider icu --icu-locale en-US -D data &&
pg_ctl -D data -l logfile start

1) The ECPG tests fail because they use the SQL_ASCII encoding [2], the 
database template0 uses the ICU locale provider and SQL_ASCII is not 
supported by ICU:

$ make -C src/interfaces/ecpg/ installcheck
...
============== creating database "ecpg1_regression"   ==============
ERROR:  encoding "SQL_ASCII" is not supported with ICU provider
ERROR:  database "ecpg1_regression" does not exist
command failed: "/home/marina/postgresql/master/my/inst/bin/psql" -X -c 
"CREATE DATABASE \"ecpg1_regression\" TEMPLATE=template0 
ENCODING='SQL_ASCII'" -c "ALTER DATABASE \"ecpg1_regression\" SET 
lc_messages TO 'C';ALTER DATABASE \"ecpg1_regression\" SET lc_monetary 
TO 'C';ALTER DATABASE \"ecpg1_regression\" SET lc_numeric TO 'C';ALTER 
DATABASE \"ecpg1_regression\" SET lc_time TO 'C';ALTER DATABASE 
\"ecpg1_regression\" SET bytea_output TO 'hex';ALTER DATABASE 
\"ecpg1_regression\" SET timezone_abbreviations TO 'Default';" 
"postgres"

2) The option --no-locale in pg_regress is described as "use C locale" 
[3]. But in this case the created databases actually use the ICU locale 
provider with the ICU cluster locale from template0 (see 
diff_check_backend_used_provider.txt):

$ make NO_LOCALE=1 installcheck

In regression.diffs:

diff -U3 
/home/marina/postgresql/master/src/test/regress/expected/test_setup.out 
/home/marina/postgresql/master/src/test/regress/results/test_setup.out
--- 
/home/marina/postgresql/master/src/test/regress/expected/test_setup.out  
   2022-09-27 05:31:27.674628815 +0300
+++ 
/home/marina/postgresql/master/src/test/regress/results/test_setup.out   
  2022-10-21 15:09:31.232992885 +0300
@@ -143,6 +143,798 @@
  \set filename :abs_srcdir '/data/person.data'
  COPY person FROM :'filename';
  VACUUM ANALYZE person;
+NOTICE:  varstrfastcmp_locale sss->collate_c 0 sss->locale 0xefacd0
+NOTICE:  varstrfastcmp_locale sss->locale->provider i
+NOTICE:  varstrfastcmp_locale sss->locale->info.icu.locale en-US
...

The patch 
v1-0001-Fix-database-creation-during-installchecks-for-IC.patch fixes 
both issues for me.

[1] 
https://www.postgresql.org/message-id/727b5d5160f845dcf5e0818e625a6e56%40postgrespro.ru
[2] 

https://github.com/postgres/postgres/blob/ce20f8b9f4354b46b40fd6ebf7ce5c37d08747e0/src/interfaces/ecpg/test/Makefile#L18
[3] 
https://github.com/postgres/postgres/blob/ce20f8b9f4354b46b40fd6ebf7ce5c37d08747e0/src/test/regress/pg_regress.c#L1992

-- 
Marina Polyakova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company