change regexp_substr first argument make tests more easier to understand.

hi.
https://git.postgresql.org/cgit/postgresql.git/tree/src/test/regress/expected/strings.out#n928

SELECT regexp_substr('abcabcabc', 'a.c');
SELECT regexp_substr('abcabcabc', 'a.c', 2);
SELECT regexp_substr('abcabcabc', 'a.c', 1, 3);
SELECT regexp_substr('abcabcabc', 'a.c', 1, 4) IS NULL AS t;
SELECT regexp_substr('abcabcabc', 'A.C', 1, 2, 'i');

they all return 'abc', there are 3 'abc ' in string 'abcabcabc'
except IS NULL query.
maybe we can change regexp_substr first argument from "abcabcabc" to
"abcaXcaYc".
so the result would be more easier to understand.