Fix flaky Hypercore join test

The join test could sometimes pick a seqscan+sort instead of an
indexscan when doing a MergeAppend+MergeJoin. Disabling seqscan should
make it deterministic.

tsl/test/expected/hypercore_join.out

@@ -108,6 +108,8 @@ select format('%I.%I', chunk_schema, chunk_name)::regclass as chunk2
 -- We disable columnar scan for these tests since we have a dedicated
 -- test for this.
 set timescaledb.enable_columnarscan to false;
+-- Discourage seqscan to make sure we predictibly use indexscan
+set enable_seqscan to false;
 set enable_memoize to false;
 -- Create a hypercore with a few rows and use the big table to join
 -- with it. This should put the hypercore as the inner relation and

tsl/test/sql/hypercore_join.sql

@@ -7,6 +7,8 @@
 -- We disable columnar scan for these tests since we have a dedicated
 -- test for this.
 set timescaledb.enable_columnarscan to false;
+-- Discourage seqscan to make sure we predictibly use indexscan
+set enable_seqscan to false;
 
 set enable_memoize to false;
 

tsl/test/sql/include/hypercore_join_test.sql

@@ -37,4 +37,3 @@ where r.device_id is distinct from e.device_id;
 
 drop table :inner;
 drop table :outer;
-