Fix timing-sensitive regression test result I just created :-( --- the

DROP USER at the end of the cluster.sql test could fail, if the temp
table created in the previous session hadn't finished getting dropped.
Unluckily, I didn't see this in several repetitions of the parallel
regression tests, but it's popping up on quite a few buildfarm machines.

src/test/regress/expected/cluster.out

@@ -446,6 +446,7 @@ select * from clstr_temp;
     2 | two
 (2 rows)
 
+drop table clstr_temp;
 -- clean up
 \c -
 DROP TABLE clustertest;

src/test/regress/sql/cluster.sql

@@ -192,6 +192,7 @@ create temp table clstr_temp (col1 int primary key, col2 text);
 insert into clstr_temp values (2, 'two'), (1, 'one');
 cluster clstr_temp using clstr_temp_pkey;
 select * from clstr_temp;
+drop table clstr_temp;
 
 -- clean up
 \c -