Force hash joins to be enabled in the hash join regression tests.

Otherwise the regressplans.sh tests generate extremely slow nested
loop joins.  Back-patch to 11 where the hash join tests came in.

Reported-by: Michael Paquier
Discussion: https://postgr.es/m/20190708055256.GB2709%40paquier.xyz

src/test/regress/expected/join_hash.out

@@ -4,6 +4,7 @@
 begin;
 set local min_parallel_table_scan_size = 0;
 set local parallel_setup_cost = 0;
+set local enable_hashjoin = on;
 -- Extract bucket and batch counts from an explain analyze plan.  In
 -- general we can't make assertions about how many batches (or
 -- buckets) will be required because it can vary, but we can in some

src/test/regress/sql/join_hash.sql

@@ -6,6 +6,7 @@ begin;
 
 set local min_parallel_table_scan_size = 0;
 set local parallel_setup_cost = 0;
+set local enable_hashjoin = on;
 
 -- Extract bucket and batch counts from an explain analyze plan.  In
 -- general we can't make assertions about how many batches (or