Provide stable test for NULL-values in KNN SP-GiST

f5f084fc has removed test because of its instability.  This commit provides
alternative test with determined ordering using extra ORDER BY expression.

Backpatch-through: 12

src/test/regress/expected/create_index_spgist.out

@@ -555,6 +555,16 @@ WHERE seq.dist IS DISTINCT FROM idx.dist;
 ---+------+---+---+------+---
 (0 rows)
 
+-- check ORDER BY distance to NULL
+SELECT (SELECT p FROM kd_point_tbl ORDER BY p <-> pt, p <-> '0,0' LIMIT 1)
+FROM (VALUES (point '1,2'), (NULL), ('1234,5678')) pts(pt);
+      p      
+-------------
+ (59,21)
+ (59,21)
+ (1239,5647)
+(3 rows)
+
 EXPLAIN (COSTS OFF)
 SELECT count(*) FROM radix_text_tbl WHERE t = 'P0123456789abcdef';
                          QUERY PLAN                         

src/test/regress/sql/create_index_spgist.sql

@@ -225,6 +225,10 @@ SELECT * FROM quad_point_tbl_ord_seq3 seq FULL JOIN kd_point_tbl_ord_idx3 idx
 ON seq.n = idx.n
 WHERE seq.dist IS DISTINCT FROM idx.dist;
 
+-- check ORDER BY distance to NULL
+SELECT (SELECT p FROM kd_point_tbl ORDER BY p <-> pt, p <-> '0,0' LIMIT 1)
+FROM (VALUES (point '1,2'), (NULL), ('1234,5678')) pts(pt);
+
 
 EXPLAIN (COSTS OFF)
 SELECT count(*) FROM radix_text_tbl WHERE t = 'P0123456789abcdef';