Better fix for sequence access in hot standby test

The purpose of the test was to check access to the sequence relation on
a hot standby, so change the test to read a different column from the
sequence, instead of just reading the catalog.

From: Andreas Karlsson <andreas@proxel.se>

src/test/regress/expected/hs_standby_allowed.out

@@ -26,10 +26,10 @@ COPY hs1 TO '/tmp/copy_test';
 \! cat /tmp/copy_test
 1
 -- Access sequence directly
-select min_value as sequence_min_value from pg_sequences where sequencename = 'hsseq';
- sequence_min_value 
---------------------
-                  1
+select is_called from hsseq;
+ is_called 
+-----------
+ f
 (1 row)
 
 -- Transactions

src/test/regress/sql/hs_standby_allowed.sql

@@ -16,7 +16,7 @@ COPY hs1 TO '/tmp/copy_test';
 \! cat /tmp/copy_test
 
 -- Access sequence directly
-select min_value as sequence_min_value from pg_sequences where sequencename = 'hsseq';
+select is_called from hsseq;
 
 -- Transactions