Stabilize ordered_append test

Plan switches from Index Only Scan to Index Scan suggest visibility
map race condition. Adding FREEZE to the VACUUM command makes sure
visibility map is up to date and reduces plan time inconsistencies.

Author: antekresic

tsl/test/sql/include/ordered_append.sql

@@ -11,7 +11,7 @@
 
 SET timescaledb.enable_decompression_sorted_merge = 0;
 
-vacuum analyze :TEST_TABLE;
+vacuum (analyze, freeze) :TEST_TABLE;
 
 -- test ASC for ordered chunks
 :PREFIX