Adjust unstable regression test case.

Test queries added by commit 69835bc89 are giving unexpected results
on some smaller buildfarm critters.  I think probably the seqscan
logic is kicking in to cause the scans to not start at the beginning
of the table.  Add ORDER BY to make them be indexscans instead.

Per buildfarm member chipmunk.

src/test/regress/expected/psql.out

@@ -3161,7 +3161,7 @@ last error message: syntax error at end of input
 last error code: 42601
 -- check row count for a cursor-fetched query
 \set FETCH_COUNT 10
-select unique2 from tenk1 limit 19;
+select unique2 from tenk1 order by unique2 limit 19;
  unique2 
 ---------
        0
@@ -3191,8 +3191,20 @@ error: false
 error code: 00000
 \echo 'number of rows:' :ROW_COUNT
 number of rows: 19
--- cursor-fetched query with an error
-select 1/unique1 from tenk1;
+-- cursor-fetched query with an error after the first group
+select 1/(15-unique2) from tenk1 order by unique2 limit 19;
+ ?column? 
+----------
+        0
+        0
+        0
+        0
+        0
+        0
+        0
+        0
+        0
+        0
 ERROR:  division by zero
 \echo 'error:' :ERROR
 error: true

src/test/regress/sql/psql.sql

@@ -656,13 +656,13 @@ SELECT 4 AS \gdesc
 
 -- check row count for a cursor-fetched query
 \set FETCH_COUNT 10
-select unique2 from tenk1 limit 19;
+select unique2 from tenk1 order by unique2 limit 19;
 \echo 'error:' :ERROR
 \echo 'error code:' :SQLSTATE
 \echo 'number of rows:' :ROW_COUNT
 
--- cursor-fetched query with an error
-select 1/unique1 from tenk1;
+-- cursor-fetched query with an error after the first group
+select 1/(15-unique2) from tenk1 order by unique2 limit 19;
 \echo 'error:' :ERROR
 \echo 'error code:' :SQLSTATE
 \echo 'number of rows:' :ROW_COUNT