Specify locale provider for pg_regress --no-locale

pg_regress has a --no-locale option that forces the temporary database to
have C locale.  However, currently, locale C only exists in the 'builtin'
locale provider.  This makes 'pg_regress --no-locale' fail when the default
locale provider is not 'builtin'.  This commit makes 'pg_regress --no-locale'
specify both LOCALE='C' and LOCALE_PROVIDER='builtin'.

Discussion: https://postgr.es/m/b54921f95e23b4391b1613e9053a3d58%40postgrespro.ru
Author: Oleg Tselebrovskiy <o.tselebrovskiy@postgrespro.ru>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>

src/test/regress/pg_regress.c

@@ -1968,10 +1968,10 @@ create_database(const char *dbname)
 	 */
 	if (encoding)
 		psql_add_command(buf, "CREATE DATABASE \"%s\" TEMPLATE=template0 ENCODING='%s'%s", dbname, encoding,
-						 (nolocale) ? " LOCALE='C'" : "");
+						 (nolocale) ? " LOCALE='C' LOCALE_PROVIDER='builtin'" : "");
 	else
 		psql_add_command(buf, "CREATE DATABASE \"%s\" TEMPLATE=template0%s", dbname,
-						 (nolocale) ? " LOCALE='C'" : "");
+						 (nolocale) ? " LOCALE='C' LOCALE_PROVIDER='builtin'" : "");
 	psql_add_command(buf,
 					 "ALTER DATABASE \"%s\" SET lc_messages TO 'C';"
 					 "ALTER DATABASE \"%s\" SET lc_monetary TO 'C';"