Adjust regex for test with opening parenthesis in character classes

As written, the test was throwing an error because of an unbalanced
parenthesis.  The regex used in the test is adjusted to not fail and to
test the case of an opening parenthesis in a character class after some
nested square brackets.

Oversight in d46911e584.

Discussion: https://postgr.es/m/16ab039d1af455652bdf4173402ddda145f2c73b.camel@cybertec.at

src/test/regress/expected/strings.out

@@ -606,8 +606,13 @@ EXPLAIN (COSTS OFF) SELECT * FROM TEXT_TBL WHERE f1 SIMILAR TO '$[$[:alnum:]$]$'
 (2 rows)
 
 -- Opening parenthesis "("
-EXPLAIN (COSTS OFF) SELECT * FROM TEXT_TBL WHERE f1 SIMILAR TO '([([:alnum:](](';
-ERROR:  invalid regular expression: parentheses () not balanced
+EXPLAIN (COSTS OFF) SELECT * FROM TEXT_TBL WHERE f1 SIMILAR TO '()[([:alnum:](]()';
+                      QUERY PLAN                      
+------------------------------------------------------
+ Seq Scan on text_tbl
+   Filter: (f1 ~ '^(?:(?:)[([:alnum:](](?:))$'::text)
+(2 rows)
+
 -- Caret "^"
 EXPLAIN (COSTS OFF) SELECT * FROM TEXT_TBL WHERE f1 SIMILAR TO '^[^[:alnum:]^[^^][[^^]][\^][[\^]]\^]^';
                                QUERY PLAN                               

src/test/regress/sql/strings.sql

@@ -198,7 +198,7 @@ EXPLAIN (COSTS OFF) SELECT * FROM TEXT_TBL WHERE f1 SIMILAR TO '.[.[:alnum:].].'
 -- Dollar "$"
 EXPLAIN (COSTS OFF) SELECT * FROM TEXT_TBL WHERE f1 SIMILAR TO '$[$[:alnum:]$]$';
 -- Opening parenthesis "("
-EXPLAIN (COSTS OFF) SELECT * FROM TEXT_TBL WHERE f1 SIMILAR TO '([([:alnum:](](';
+EXPLAIN (COSTS OFF) SELECT * FROM TEXT_TBL WHERE f1 SIMILAR TO '()[([:alnum:](]()';
 -- Caret "^"
 EXPLAIN (COSTS OFF) SELECT * FROM TEXT_TBL WHERE f1 SIMILAR TO '^[^[:alnum:]^[^^][[^^]][\^][[\^]]\^]^';
 -- Closing square bracket "]" at the beginning of character class