Add SQL_ASCII encoding test case

src/test/regress/sql/sql_ascii.sql

+drop table computer_terms;
+create table computer_terms(term text, category text, comments char(16));
+create index computer_terms_index1 on computer_terms using btree(term);
+create index computer_terms_index2 on computer_terms using btree(category);
+insert into computer_terms values('computer display', 'X-A01-Y', 'a comment 1');
+insert into computer_terms values('computer graphics', 'T-B01-Y', 'a comment 2');
+insert into computer_terms values('computer programmer', 'S-Z01-Y', 'a comment 3');
+vacuum computer_terms;
+select * from computer_terms;
+select * from computer_terms where category = 'X-A01-Y';
+select * from computer_terms where category ~* 'x-a01-y';
+select * from computer_terms where category like '_-A01-_';
+select * from computer_terms where category like '_-A%';
+select * from computer_terms where term ~ 'computer [dg]';
+select * from computer_terms where term ~* 'computer [DG]';
+select *,character_length(term) from computer_terms;
+select *,octet_length(term) from computer_terms;
+select *,position('s' in term) from computer_terms;
+select *,substring(term from 10 for 4) from computer_terms;